Thanks to marcusklaas' hard work in https://github.com/raphlinus/pulldown-cmark/pull/469, this fixes a lot of rustdoc bugs!

- Get rid of unnecessary `RefCell`
- Fix duplicate warnings for broken implicit reference link
- Remove unnecessary copy of links

this is to avoid compiling multiple version of the crates in rustc

Use std::io::{Read, Write} and {to, from}_{le, be}_bytes methods in
order to remove byteorder from librustc_middle's dependency graph.

Co-authored-by: NOliver Scherer <github35764891676564198441@oli-obk.de>

The only change is that `expect_file` now uses path relative to the
current file (same as `include!`). Before, it used paths relative to
the workspace root, which makes no sense.

StringReader is an intornal abstraction which at the moment changes a
lot, so these unit tests cause quite a bit of friction.

Moving them to rustc_lexer and more ingerated-testing style should
make them much less annoying, hopefully without decreasing their
usefulness much.

Note that coloncolon tests are removed (it's unclear what those are
testing).

\r\n tests are removed as well, as we normalize line endings even
before lexing.

Fixes #57820 and #46651

It's a unit-test in a sense that it only checks syntax highlighting.
However, the resulting HTML is written to disk and can be easily
inspected in the browser.

To update the test, run with `--bless` argument or set
`UPDATE_EXPEC=1` env var

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.