This commit adds a new target to the compiler: wasm32-unknown-unknown. This
target is a reimagining of what it looks like to generate WebAssembly code from
Rust. Instead of using Emscripten which can bring with it a weighty runtime this
instead is a target which uses only the LLVM backend for WebAssembly and a
"custom linker" for now which will hopefully one day be direct calls to lld.

Notable features of this target include:

* There is zero runtime footprint. The target assumes nothing exists other than
  the wasm32 instruction set.
* There is zero toolchain footprint beyond adding the target. No custom linker
  is needed, rustc contains everything.
* Very small wasm modules can be generated directly from Rust code using this
  target.
* Most of the standard library is stubbed out to return an error, but anything
  related to allocation works (aka `HashMap`, `Vec`, etc).
* Naturally, any `#[no_std]` crate should be 100% compatible with this new
  target.

This target is currently somewhat janky due to how linking works. The "linking"
is currently unconditional whole program LTO (aka LLVM is being used as a
linker). Naturally that means compiling programs is pretty slow! Eventually
though this target should have a linker.

This target is also intended to be quite experimental. I'm hoping that this can
act as a catalyst for further experimentation in Rust with WebAssembly. Breaking
changes are very likely to land to this target, so it's not recommended to rely
on it in any critical capacity yet. We'll let you know when it's "production
ready".

---

Currently testing-wise this target is looking pretty good but isn't complete.
I've got almost the entire `run-pass` test suite working with this target (lots
of tests ignored, but many passing as well). The `core` test suite is still
getting LLVM bugs fixed to get that working and will take some time. Relatively
simple programs all seem to work though!

---

It's worth nothing that you may not immediately see the "smallest possible wasm
module" for the input you feed to rustc. For various reasons it's very difficult
to get rid of the final "bloat" in vanilla rustc (again, a real linker should
fix all this). For now what you'll have to do is:

    cargo install --git https://github.com/alexcrichton/wasm-gc
    wasm-gc foo.wasm bar.wasm

And then `bar.wasm` should be the smallest we can get it!

---

In any case for now I'd love feedback on this, particularly on the various
integration points if you've got better ideas of how to approach them!

This commit updates the bootstrap compiler and clears out a number
of #[cfg(stage0)] annotations and related business

In https://github.com/rust-lang/rust/pull/42938 we made the compiler
emit a call to begin_panic_new in order to pass column info to it. Now
with stage0 updated (https://github.com/rust-lang/rust/pull/43320),
we can safely change begin_panic and start emitting calls for it again.

Stabilizes:

* `compile_error!` as a macro defined by rustc

Closes #40872

Replaced by adding extra imports, adding hidden code (`# ...`), modifying
examples to be runnable (sorry Homura), specifying non-Rust code, and
converting to should_panic, no_run, or compile_fail.

Remaining "```ignore"s received an explanation why they are being ignored.

Related to #40872

`compiler-plugins.html` is moved into the Unstable Book.
Explanation is slightly modified to match the change.

These were found by running tidy on stable versions of rust and finding
features stabilised with the wrong version numbers.

 * Factor out the nigh-identical bodies of `_print` and `_eprint` to a helper
   function `print_to` (I was sorely tempted to call it `_doprnt`).
 * Update the issue number for the unstable `eprint` feature.
 * Add entries to the "unstable book" for `eprint` and `eprint_internal`.
 * Style corrections to the documentation.

These are exactly the same as `print!` and `println!` except that
they write to stderr instead of stdout.  Issue #39228.

This affects the book, some missed things in the reference, the grammar,
and the standard library. Whew!

Removes the `STATUSES` static which duplicates truth from the pattern
match in `collect_lang_features`.

Fixes existing duplicates by renaming:
- never_type{,_impls} on `impl`s on `!`
- concat_idents{,_macro} on `macro_rules! concat_idents`

Fixes #37013.

Fixes #34697

This is mostly just a documentation fix as I don't think stability
attributes have any effect on macros.

This commit is an implementation of [RFC 1513] which allows applications to
alter the behavior of panics at compile time. A new compiler flag, `-C panic`,
is added and accepts the values `unwind` or `panic`, with the default being
`unwind`. This model affects how code is generated for the local crate, skipping
generation of landing pads with `-C panic=abort`.

[RFC 1513]: https://github.com/rust-lang/rfcs/blob/master/text/1513-less-unwinding.md

Panic implementations are then provided by crates tagged with
`#![panic_runtime]` and lazily required by crates with
`#![needs_panic_runtime]`. The panic strategy (`-C panic` value) of the panic
runtime must match the final product, and if the panic strategy is not `abort`
then the entire DAG must have the same panic strategy.

With the `-C panic=abort` strategy, users can expect a stable method to disable
generation of landing pads, improving optimization in niche scenarios,
decreasing compile time, and decreasing output binary size. With the `-C
panic=unwind` strategy users can expect the existing ability to isolate failure
in Rust code from the outside world.

Organizationally, this commit dismantles the `sys_common::unwind` module in
favor of some bits moving part of it to `libpanic_unwind` and the rest into the
`panicking` module in libstd. The custom panic runtime support is pretty similar
to the custom allocator support with the only major difference being how the
panic runtime is injected (takes the `-C panic` flag into account).

Right now everything in TARGET_CRATES is built by default for all non-fulldeps
tests and is distributed by default for all target standard library packages.
Currenly this includes a number of unstable crates which are rarely used such as
`graphviz` and `rbml`>

This commit trims down the set of `TARGET_CRATES`, moves a number of tests to
`*-fulldeps` as a result, and trims down the dependencies of libtest so we can
distribute fewer crates in the `rust-std` packages.