Snapshot testing is a technique for writing maintainable unit tests.
Unlike usual `assert_eq!` tests, snapshot tests allow
to *automatically* upgrade expected values on test failure.
In a sense, snapshot tests are inline-version of our beloved
UI-tests.

Example:

![expect](https://user-images.githubusercontent.com/1711539/90888810-3bcc8180-e3b7-11ea-9626-d06e89e1a0bb.gif)

A particular library we use, `expect_test` provides an `expect!`
macro, which creates a sort of self-updating string literal (by using
`file!` macro). Self-update is triggered by setting `UPDATE_EXPECT`
environmental variable (this info is printed during the test failure).
This library was extracted from rust-analyzer, where we use it for
most of our tests.

There are some other, more popular snapshot testing libraries:

* https://github.com/mitsuhiko/insta
* https://github.com/aaronabramov/k9

The main differences of `expect` are:

* first-class snapshot objects (so, tests can be written as functions,
  rather than as macros)
* focus on inline-snapshots (but file snapshots are also supported)
* restricted feature set (only `assert_eq` and `assert_debug_eq`)
* no extra runtime (ie, no `cargo insta`)

See https://github.com/rust-analyzer/rust-analyzer/pull/5101 for a
an extended comparison.

It is unclear if this testing style will stick with rustc in the long
run. At the moment, rustc is mainly tested via integrated UI tests.
But in the library-ified world, unit-tests will become somewhat more
important (that's why use use `rustc_lexer` library-ified library as
an example in this PR). Given that the cost of removal shouldn't be
too high, it probably makes sense to just see if this flies!

Updates cargo_metadata in tidy's Cargo.toml from 0.9.1 to 0.11
Real version change 0.9.11 -> 0.11.1
https://github.com/oli-obk/cargo_metadata/compare/v0.9.1...v0.11.1

0.9.2 is a breaking change to the #[cfg(version)] test in
src/test/ui/feature-gates/feature-gate-config-version.*
Let's downgrade for now.

Per https://github.com/rust-lang/rust/pull/75555#issuecomment-675090858
Zlib license might be OK. "OR Apache-2.0 OR MIT" definitely is.
unicode-normalization depends on this and rustc_parse, clippy,
and many other things depend on unicode-normalization.

This runs cargo update, applying the following changes:
      Adding arrayref v0.3.6
      Adding base64 v0.11.0
      Adding blake2b_simd v0.5.10
      Adding cloudabi v0.1.0
      Adding crossbeam-queue v0.2.3
      Adding instant v0.1.6
      Adding lock_api v0.4.1
      Adding maybe-uninit v2.0.0
      Adding parking_lot_core v0.7.2
      Adding parking_lot_core v0.8.0
      Adding parking_lot v0.11.0
      Adding proc-macro-error-attr v1.0.4
      Adding quick-error v2.0.0
      Adding rust-argon2 v0.7.0
      Adding signal-hook-registry v1.2.1
      Adding smallvec v0.6.13
      Adding smallvec v1.4.2
      Adding tinyvec v0.3.3
    Removing argon2rs v0.2.5
    Removing arrayvec v0.4.7
    Removing blake2-rfc v0.2.18
    Removing fuchsia-cprng v0.1.1
    Removing nodrop v0.1.12
    Removing parking_lot_core v0.7.1
    Removing rand_core v0.3.0
    Removing rand_core v0.4.0
    Removing rand_os v0.1.3
    Removing rdrand v0.4.0
    Removing scoped_threadpool v0.1.9
    Removing signal-hook v0.1.7
    Removing smallvec v0.6.10
    Removing smallvec v1.4.0
    Updating aho-corasick v0.7.10 -> v0.7.13
    Updating anyhow v1.0.31 -> v1.0.32
    Updating arc-swap v0.3.7 -> v0.4.7
    Updating bitmaps v2.0.0 -> v2.1.0
    Updating bstr v0.1.3 -> v0.2.13
    Updating byteorder v1.3.2 -> v1.3.4
    Updating bytesize v1.0.0 -> v1.0.1
    Updating bytes v0.4.11 -> v0.4.12
    Updating cargo_metadata v0.8.0 -> v0.8.2
    Updating chrono v0.4.6 -> v0.4.15
    Updating clap v2.33.0 -> v2.33.3
    Updating cmake v0.1.42 -> v0.1.44
    Updating constant_time_eq v0.1.3 -> v0.1.5
    Updating crossbeam-channel v0.4.0 -> v0.4.3
    Updating crossbeam-deque v0.7.1 -> v0.7.3
    Updating crossbeam-epoch v0.7.2 -> v0.8.2
    Updating crossbeam-utils v0.6.5 -> v0.6.6
    Updating crypto-hash v0.3.1 -> v0.3.4
    Updating ctor v0.1.13 -> v0.1.15
    Updating curl-sys v0.4.25 -> v0.4.34+curl-7.71.1
    Updating curl v0.4.25 -> v0.4.31
    Updating derive_more v0.99.2 -> v0.99.9
    Updating diff v0.1.11 -> v0.1.12
    Updating directories v2.0.1 -> v2.0.2
    Updating dirs-sys v0.3.3 -> v0.3.5
    Updating dirs v2.0.1 -> v2.0.2
    Updating either v1.5.0 -> v1.6.0
    Updating failure v0.1.5 -> v0.1.8
    Updating filetime v0.2.9 -> v0.2.12
    Updating fnv v1.0.6 -> v1.0.7
    Updating fortanix-sgx-abi v0.3.2 -> v0.3.3
    Updating fst v0.3.0 -> v0.3.5
    Updating futures v0.1.28 -> v0.1.29
    Updating git2 v0.13.5 -> v0.13.8
    Updating globset v0.4.3 -> v0.4.5
    Updating handlebars v3.0.1 -> v3.4.0
    Updating heck v0.3.0 -> v0.3.1
    Updating hex v0.4.0 -> v0.4.2
    Updating home v0.5.1 -> v0.5.3
    Updating humantime v2.0.0 -> v2.0.1
    Updating ignore v0.4.11 -> v0.4.16
    Updating itertools v0.8.0 -> v0.8.2
    Updating itoa v0.4.4 -> v0.4.6
    Updating jemalloc-sys v0.3.0 -> v0.3.2
    Updating jsonrpc-client-transports v14.0.5 -> v14.2.1
    Updating jsonrpc-core-client v14.0.5 -> v14.2.0
    Updating jsonrpc-core v14.0.5 -> v14.2.0
    Updating jsonrpc-derive v14.0.5 -> v14.2.1
    Updating jsonrpc-pubsub v14.0.6 -> v14.2.0
    Updating jsonrpc-server-utils v14.0.5 -> v14.2.0
    Updating json v0.11.13 -> v0.11.15
    Updating lazycell v1.2.1 -> v1.3.0
    Updating libgit2-sys v0.12.7+1.0.0 -> v0.12.9+1.0.1
    Updating libnghttp2-sys v0.1.2 -> v0.1.4+1.41.0
    Updating libssh2-sys v0.2.14 -> v0.2.18
    Updating libz-sys v1.0.25 -> v1.0.27
    Updating linked-hash-map v0.5.2 -> v0.5.3
    Updating log v0.4.8 -> v0.4.11
    Updating lzma-sys v0.1.14 -> v0.1.16
    Updating macro-utils v0.1.2 -> v0.1.3
    Updating maplit v1.0.1 -> v1.0.2
    Updating mdbook v0.4.0 -> v0.4.2
    Updating memoffset v0.5.1 -> v0.5.5
    Updating mio-named-pipes v0.1.6 -> v0.1.7
    Updating mio-uds v0.6.7 -> v0.6.8
    Updating mio v0.6.16 -> v0.6.22
    Updating miow v0.3.3 -> v0.3.5
    Updating net2 v0.2.33 -> v0.2.34
    Updating new_debug_unreachable v1.0.3 -> v1.0.4
    Updating num_cpus v1.10.1 -> v1.13.0
    Updating num-integer v0.1.39 -> v0.1.43
    Updating num-traits v0.2.6 -> v0.2.12
    Updating once_cell v1.1.0 -> v1.4.0
    Updating opener v0.4.0 -> v0.4.1
    Updating openssl-src v111.9.0+1.1.1g -> v111.10.2+1.1.1g
    Updating openssl-sys v0.9.54 -> v0.9.58
    Updating openssl v0.10.25 -> v0.10.30
    Updating open v1.2.1 -> v1.4.0
    Updating packed_simd v0.3.1 -> v0.3.3
    Updating pest v2.1.0 -> v2.1.3
    Updating pkg-config v0.3.17 -> v0.3.18
    Updating proc-macro2 v1.0.3 -> v1.0.19
    Updating proc-macro-crate v0.1.4 -> v0.1.5
    Updating proc-macro-error v0.2.6 -> v1.0.4
    Updating psm v0.1.10 -> v0.1.11
    Updating pulldown-cmark v0.7.1 -> v0.7.2
    Updating punycode v0.4.0 -> v0.4.1
    Updating quote v1.0.2 -> v1.0.7
    Updating rayon-core v1.6.0 -> v1.7.1
    Updating rayon v1.2.0 -> v1.3.1
    Updating redox_syscall v0.1.56 -> v0.1.57
    Updating redox_users v0.3.0 -> v0.3.4
    Updating regex-syntax v0.6.17 -> v0.6.18
    Updating regex v1.3.7 -> v1.3.9
    Updating remove_dir_all v0.5.2 -> v0.5.3
    Updating rustfix v0.5.0 -> v0.5.1
    Updating ryu v1.0.0 -> v1.0.5
    Updating same-file v1.0.4 -> v1.0.6
    Updating schannel v0.1.16 -> v0.1.19
    Updating scopeguard v1.0.0 -> v1.1.0
    Updating serde_derive v1.0.106 -> v1.0.115
    Updating serde_ignored v0.1.0 -> v0.1.2
    Updating serde_json v1.0.40 -> v1.0.57
    Updating serde_repr v0.1.5 -> v0.1.6
    Updating serde v1.0.99 -> v1.0.115
    Updating shell-escape v0.1.4 -> v0.1.5
    Updating stable_deref_trait v1.1.0 -> v1.2.0
    Updating stacker v0.1.9 -> v0.1.11
    Updating structopt-derive v0.3.1 -> v0.4.9
    Updating structopt v0.3.1 -> v0.3.16
    Updating synstructure v0.12.1 -> v0.12.4
    Updating syn v1.0.11 -> v1.0.38
    Updating tar v0.4.26 -> v0.4.29
    Updating tendril v0.4.0 -> v0.4.1
    Updating term v0.6.0 -> v0.6.1
    Updating thiserror-impl v1.0.5 -> v1.0.20
    Updating thiserror v1.0.5 -> v1.0.20
    Updating time v0.1.42 -> v0.1.43
    Updating tokio-codec v0.1.1 -> v0.1.2
    Updating tokio-current-thread v0.1.6 -> v0.1.7
    Updating tokio-executor v0.1.9 -> v0.1.10
    Updating tokio-fs v0.1.6 -> v0.1.7
    Updating tokio-io v0.1.12 -> v0.1.13
    Updating tokio-process v0.2.4 -> v0.2.5
    Updating tokio-reactor v0.1.11 -> v0.1.12
    Updating tokio-signal v0.2.7 -> v0.2.9
    Updating tokio-sync v0.1.7 -> v0.1.8
    Updating tokio-tcp v0.1.3 -> v0.1.4
    Updating tokio-threadpool v0.1.17 -> v0.1.18
    Updating tokio-timer v0.2.12 -> v0.2.13
    Updating tokio-udp v0.1.5 -> v0.1.6
    Updating tokio-uds v0.2.5 -> v0.2.7
    Updating toml v0.5.3 -> v0.5.6
    Updating tracing-attributes v0.1.9 -> v0.1.10
    Updating tracing-core v0.1.12 -> v0.1.14
    Updating tracing-subscriber v0.2.10 -> v0.2.11
    Updating tracing v0.1.18 -> v0.1.19
    Updating ucd-parse v0.1.4 -> v0.1.8
    Updating ucd-trie v0.1.1 -> v0.1.3
    Updating unicode-normalization v0.1.12 -> v0.1.13
    Updating unicode-script v0.5.1 -> v0.5.2
    Updating unicode-width v0.1.6 -> v0.1.8
    Updating unicode-xid v0.2.0 -> v0.2.1
    Updating url v2.1.0 -> v2.1.1
    Updating utf-8 v0.7.2 -> v0.7.5
    Updating vcpkg v0.2.8 -> v0.2.10
    Updating vec_map v0.8.1 -> v0.8.2
    Updating version_check v0.9.1 -> v0.9.2
    Updating walkdir v2.2.7 -> v2.3.1
    Updating winapi v0.3.8 -> v0.3.9
    Updating xz2 v0.1.5 -> v0.1.6
    Updating yaml-merge-keys v0.4.0 -> v0.4.1
    Updating yaml-rust v0.4.3 -> v0.4.4

- Move the type parameter from `encode` and `decode` methods to
  the trait.
- Remove `UseSpecialized(En|De)codable` traits.
- Remove blanket impls for references.
- Add `RefDecodable` trait to allow deserializing to arena-allocated
  references safely.
- Remove ability to (de)serialize HIR.
- Create proc-macros `(Ty)?(En|De)codable` to help implement these new
  traits.

Previously if the compiler error'd, fatally, then temporary directories which
should be preserved by -Csave-temps would be deleted due to fatal compiler
errors being implemented as panics.

By moving `{known,used}_attrs` from `SessionGlobals` to `Session`. This
means they are accessed via the `Session`, rather than via TLS. A few
`Attr` methods and `librustc_ast` functions are now methods of
`Session`.

All of this required passing a `Session` to lots of functions that didn't
already have one. Some of these functions also had arguments removed, because
those arguments could be accessed directly via the `Session` argument.

`contains_feature_attr()` was dead, and is removed.

Some functions were moved from `librustc_ast` elsewhere because they now need
to access `Session`, which isn't available in that crate.
- `entry_point_type()` --> `librustc_builtin_macros`
- `global_allocator_spans()` --> `librustc_metadata`
- `is_proc_macro_attr()` --> `Session`

We store an `ImplicitCtxt` pointer in a thread-local value (TLV). This allows
implicit access to a `GlobalCtxt` and some other things.

We also store a `GlobalCtxt` pointer in `GCX_PTR`. This is always the same
`GlobalCtxt` as the one within the `ImplicitCtxt` pointer in TLV. `GCX_PTR`
is only used in the parallel compiler's `handle_deadlock()` function.

This commit does the following.
- It removes `GCX_PTR`.
- It also adds `ImplicitCtxt::new()`, which constructs an `ImplicitCtxt` from a
  `GlobalCtxt`. `ImplicitCtxt::new()` + `tls::enter_context()` is now
  equivalent to the old `tls::enter_global()`.
- Makes `tls::get_tlv()` public for the parallel compiler, because it's
  now used in `handle_deadlock()`.

This removes all dependencies on pre-1.0 proc-macro ecosystem crates
(syn, quote, and proc-macro2)

This is preparation for PR #73084

This commit is a proof-of-concept for switching the standard library's
backtrace symbolication mechanism on most platforms from libbacktrace to
gimli. The standard library's support for `RUST_BACKTRACE=1` requires
in-process parsing of object files and DWARF debug information to
interpret it and print the filename/line number of stack frames as part
of a backtrace.

Historically this support in the standard library has come from a
library called "libbacktrace". The libbacktrace library seems to have
been extracted from gcc at some point and is written in C. We've had a
lot of issues with libbacktrace over time, unfortunately, though. The
library does not appear to be actively maintained since we've had
patches sit for months-to-years without comments. We have discovered a
good number of soundness issues with the library itself, both when
parsing valid DWARF as well as invalid DWARF. This is enough of an issue
that the libs team has previously decided that we cannot feed untrusted
inputs to libbacktrace. This also doesn't take into account the
portability of libbacktrace which has been difficult to manage and
maintain over time. While possible there are lots of exceptions and it's
the main C dependency of the standard library right now.

For years it's been the desire to switch over to a Rust-based solution
for symbolicating backtraces. It's been assumed that we'll be using the
Gimli family of crates for this purpose, which are targeted at safely
and efficiently parsing DWARF debug information. I've been working
recently to shore up the Gimli support in the `backtrace` crate. As of a
few weeks ago the `backtrace` crate, by default, uses Gimli when loaded
from crates.io. This transition has gone well enough that I figured it
was time to start talking seriously about this change to the standard
library.

This commit is a preview of what's probably the best way to integrate
the `backtrace` crate into the standard library with the Gimli feature
turned on. While today it's used as a crates.io dependency, this commit
switches the `backtrace` crate to a submodule of this repository which
will need to be updated manually. This is not done lightly, but is
thought to be the best solution. The primary reason for this is that the
`backtrace` crate needs to do some pretty nontrivial filesystem
interactions to locate debug information. Working without `std::fs` is
not an option, and while it might be possible to do some sort of
trait-based solution when prototyped it was found to be too unergonomic.
Using a submodule allows the `backtrace` crate to build as a submodule
of the `std` crate itself, enabling it to use `std::fs` and such.

Otherwise this adds new dependencies to the standard library. This step
requires extra attention because this means that these crates are now
going to be included with all Rust programs by default. It's important
to note, however, that we're already shipping libbacktrace with all Rust
programs by default and it has a bunch of C code implementing all of
this internally anyway, so we're basically already switching
already-shipping functionality to Rust from C.

* `object` - this crate is used to parse object file headers and
  contents. Very low-level support is used from this crate and almost
  all of it is disabled. Largely we're just using struct definitions as
  well as convenience methods internally to read bytes and such.

* `addr2line` - this is the main meat of the implementation for
  symbolication. This crate depends on `gimli` for DWARF parsing and
  then provides interfaces needed by the `backtrace` crate to turn an
  address into a filename / line number. This crate is actually pretty
  small (fits in a single file almost!) and mirrors most of what
  `dwarf.c` does for libbacktrace.

* `miniz_oxide` - the libbacktrace crate transparently handles
  compressed debug information which is compressed with zlib. This crate
  is used to decompress compressed debug sections.

* `gimli` - not actually used directly, but a dependency of `addr2line`.

* `adler32`- not used directly either, but a dependency of
  `miniz_oxide`.

The goal of this change is to improve the safety of backtrace
symbolication in the standard library, especially in the face of
possibly malformed DWARF debug information. Even to this day we're still
seeing segfaults in libbacktrace which could possibly become security
vulnerabilities. This change should almost entirely eliminate this
possibility whilc also paving the way forward to adding more features
like split debug information.

Some references for those interested are:

* Original addition of libbacktrace - #12602
* OOM with libbacktrace - #24231
* Backtrace failure due to use of uninitialized value - #28447
* Possibility to feed untrusted data to libbacktrace - #21889
* Soundness fix for libbacktrace - #33729
* Crash in libbacktrace - #39468
* Support for macOS, never merged - ianlancetaylor/libbacktrace#2
* Performance issues with libbacktrace - #29293, #37477
* Update procedure is quite complicated due to how many patches we
  need to carry - #50955
* Libbacktrace doesn't work on MinGW with dynamic libs - #71060
* Segfault in libbacktrace on macOS - #71397

Switching to Rust will not make us immune to all of these issues. The
crashes are expected to go away, but correctness and performance may
still have bugs arise. The gimli and `backtrace` crates, however, are
actively maintained unlike libbacktrace, so this should enable us to at
least efficiently apply fixes as situations come up.

This reverts commit 13db3cc1.

This commit is a proof-of-concept for switching the standard library's
backtrace symbolication mechanism on most platforms from libbacktrace to
gimli. The standard library's support for `RUST_BACKTRACE=1` requires
in-process parsing of object files and DWARF debug information to
interpret it and print the filename/line number of stack frames as part
of a backtrace.

Historically this support in the standard library has come from a
library called "libbacktrace". The libbacktrace library seems to have
been extracted from gcc at some point and is written in C. We've had a
lot of issues with libbacktrace over time, unfortunately, though. The
library does not appear to be actively maintained since we've had
patches sit for months-to-years without comments. We have discovered a
good number of soundness issues with the library itself, both when
parsing valid DWARF as well as invalid DWARF. This is enough of an issue
that the libs team has previously decided that we cannot feed untrusted
inputs to libbacktrace. This also doesn't take into account the
portability of libbacktrace which has been difficult to manage and
maintain over time. While possible there are lots of exceptions and it's
the main C dependency of the standard library right now.

For years it's been the desire to switch over to a Rust-based solution
for symbolicating backtraces. It's been assumed that we'll be using the
Gimli family of crates for this purpose, which are targeted at safely
and efficiently parsing DWARF debug information. I've been working
recently to shore up the Gimli support in the `backtrace` crate. As of a
few weeks ago the `backtrace` crate, by default, uses Gimli when loaded
from crates.io. This transition has gone well enough that I figured it
was time to start talking seriously about this change to the standard
library.

This commit is a preview of what's probably the best way to integrate
the `backtrace` crate into the standard library with the Gimli feature
turned on. While today it's used as a crates.io dependency, this commit
switches the `backtrace` crate to a submodule of this repository which
will need to be updated manually. This is not done lightly, but is
thought to be the best solution. The primary reason for this is that the
`backtrace` crate needs to do some pretty nontrivial filesystem
interactions to locate debug information. Working without `std::fs` is
not an option, and while it might be possible to do some sort of
trait-based solution when prototyped it was found to be too unergonomic.
Using a submodule allows the `backtrace` crate to build as a submodule
of the `std` crate itself, enabling it to use `std::fs` and such.

Otherwise this adds new dependencies to the standard library. This step
requires extra attention because this means that these crates are now
going to be included with all Rust programs by default. It's important
to note, however, that we're already shipping libbacktrace with all Rust
programs by default and it has a bunch of C code implementing all of
this internally anyway, so we're basically already switching
already-shipping functionality to Rust from C.

* `object` - this crate is used to parse object file headers and
  contents. Very low-level support is used from this crate and almost
  all of it is disabled. Largely we're just using struct definitions as
  well as convenience methods internally to read bytes and such.

* `addr2line` - this is the main meat of the implementation for
  symbolication. This crate depends on `gimli` for DWARF parsing and
  then provides interfaces needed by the `backtrace` crate to turn an
  address into a filename / line number. This crate is actually pretty
  small (fits in a single file almost!) and mirrors most of what
  `dwarf.c` does for libbacktrace.

* `miniz_oxide` - the libbacktrace crate transparently handles
  compressed debug information which is compressed with zlib. This crate
  is used to decompress compressed debug sections.

* `gimli` - not actually used directly, but a dependency of `addr2line`.

* `adler32`- not used directly either, but a dependency of
  `miniz_oxide`.

The goal of this change is to improve the safety of backtrace
symbolication in the standard library, especially in the face of
possibly malformed DWARF debug information. Even to this day we're still
seeing segfaults in libbacktrace which could possibly become security
vulnerabilities. This change should almost entirely eliminate this
possibility whilc also paving the way forward to adding more features
like split debug information.

Some references for those interested are:

* Original addition of libbacktrace - #12602
* OOM with libbacktrace - #24231
* Backtrace failure due to use of uninitialized value - #28447
* Possibility to feed untrusted data to libbacktrace - #21889
* Soundness fix for libbacktrace - #33729
* Crash in libbacktrace - #39468
* Support for macOS, never merged - ianlancetaylor/libbacktrace#2
* Performance issues with libbacktrace - #29293, #37477
* Update procedure is quite complicated due to how many patches we
  need to carry - #50955
* Libbacktrace doesn't work on MinGW with dynamic libs - #71060
* Segfault in libbacktrace on macOS - #71397

Switching to Rust will not make us immune to all of these issues. The
crashes are expected to go away, but correctness and performance may
still have bugs arise. The gimli and `backtrace` crates, however, are
actively maintained unlike libbacktrace, so this should enable us to at
least efficiently apply fixes as situations come up.

rustc now generates the coverage map and can support (limited)
coverage report generation, at the function level.

Example:

$ BUILD=$HOME/rust/build/x86_64-unknown-linux-gnu
$ $BUILD/stage1/bin/rustc -Zinstrument-coverage \
$HOME/rust/src/test/run-make-fulldeps/instrument-coverage/main.rs
$ LLVM_PROFILE_FILE="main.profraw" ./main
called
$ $BUILD/llvm/bin/llvm-profdata merge -sparse main.profraw -o main.profdata
$ $BUILD/llvm/bin/llvm-cov show --instr-profile=main.profdata main
    1|      1|pub fn will_be_called() {
    2|      1|    println!("called");
    3|      1|}
    4|       |
    5|      0|pub fn will_not_be_called() {
    6|      0|    println!("should not have been called");
    7|      0|}
    8|       |
    9|      1|fn main() {
   10|      1|    let less = 1;
   11|      1|    let more = 100;
   12|      1|
   13|      1|    if less < more {
   14|      1|        will_be_called();
   15|      1|    } else {
   16|      1|        will_not_be_called();
   17|      1|    }
   18|      1|}

This commit refactors the initial implementation to fit into std and
makes some other changes:

- use MaybeUninit internally in SyncOnceCell
- correctly impl Drop for lazy::Once
- port Lazy::take from once_cell from: https://github.com/matklad/once_cell/pull/100Co-Authored-By: NPaul Dicker <pitdicker@users.noreply.github.com>