This commit is the final step in the libstd facade, #13851. The purpose of this
commit is to move libsync underneath the standard library, behind the facade.
This will allow core primitives like channels, queues, and atomics to all live
in the same location.

There were a few notable changes and a few breaking changes as part of this
movement:

* The `Vec` and `String` types are reexported at the top level of libcollections
* The `unreachable!()` macro was copied to libcore
* The `std::rt::thread` module was moved to librustrt, but it is still
  reexported at the same location.
* The `std::comm` module was moved to libsync
* The `sync::comm` module was moved under `sync::comm`, and renamed to `duplex`.
  It is now a private module with types/functions being reexported under
  `sync::comm`. This is a breaking change for any existing users of duplex
  streams.
* All concurrent queues/deques were moved directly under libsync. They are also
  all marked with #![experimental] for now if they are public.
* The `task_pool` and `future` modules no longer live in libsync, but rather
  live under `std::sync`. They will forever live at this location, but they may
  move to libsync if the `std::task` module moves as well.

[breaking-change]

When generating documentation, rustdoc has the ability to generate relative
links within the current distribution of crates to one another. To do this, it
must recognize when a crate's documentation is in the same output directory. The
current threshold for "local documentation for crate X being available" is
whether the directory "doc/X" exists.

This change modifies the build system to have new dependencies for each
directory of upstream crates for a rustdoc invocation. This will ensure that
when building documentation that all the crates in the standard distribution are
guaranteed to have relative links to one another.

This change is prompted by guaranteeing that offline docs always work with one
another. Before this change, races could mean that some docs were built before
others, and hence may have http links when relative links would suffice.

Closes #14747

**Update**

I've reimplemented this using `Cell` and `RefCell`, as suggested by @alexcrichton. By taking care with the duration of the borrows, I was able to maintain the recursive allocation feature (now covered by a test) without the use of `Unsafe`, and without breaking the non-aliasing `&mut` invariant.

**Original**

Changes both `Arena` and `TypedArena` to contain an inner struct wrapped in a `Unsafe`, and change field access to go through those instead of transmuting `&self` to `&mut self`.

Part of #13933

Fixes #8315
Fixes #11940

This uncovered some dead code, most notably in middle/liveness.rs, which I think suggests there must be something fishy with that part of the code.

The #[allow(dead_code)] annotations on some of the fields I am not super happy about but as I understand, marker type may disappear at some point.

r? @alexcrichton

Closes #14611 (std: Remove the as_utf16_p functions)
Closes #14694 (Num cleanup)
Closes #14760 (Add --color to test binary options)
Closes #14763 (std: Move dynamic_lib from std::unstable to std)
Closes #14766 (Add test for issue #13446)
Closes #14769 (collections: Add missing Default impls)
Closes #14773 (General nits)
Closes #14776 (rustdoc: Correctly classify enums/typedefs)

Both of these items are surfaced as a DefTy, so some extra logic was needed in
the decoder module to figure out whether one is actually an enum or whether it's
a typedef.

Closes #14757

Fixes #11576 by making the code never run (and hence never
pass when the test was marked `should_fail`).

This refers to green, which (AFAICT) has everything implemented. In
particular, this will help guide people to get working signal handling
via libgreen.

Add Default impls for TreeMap, TreeSet, SmallIntMap, BitvSet, DList,
PriorityQueue, RingBuf, TrieMap, and TrieSet.

Closes #13446

This leaves a deprecated reexport in place temporarily.

Closes #1457.

It uses the same behavior as rustc's.

These functions are all much better expressed via RAII using the to_utf16()
method on strings. This refactoring also takes this opportunity to properly
handle when filenames aren't valid unicode when passed through to the windows
I/O layer by properly returning I/O errors.

All previous users of the `as_utf16_p` or `as_utf16_mut_p` functions will need
to convert their code to using `foo.to_utf16().append_one(0)` to get a
null-terminated utf16 string.

[breaking-change]

gate.

This is part of unboxed closures.

r? @nick29581

This implements the design in rust-lang/rfcs#86.  It shouldn't be merged until that RFC is accepted, but it would be great if somebody has time to review the code before then.

Do this to avoid warnings on post-stage0 builds.

See RFC 22.

[breaking-change]

Apparently one of the linux bots doesn't have the USER variable defined, and
this fix will likely land more quickly than a fix to the bots.

When generating documentation, rustdoc has the ability to generate relative
links within the current distribution of crates to one another. To do this, it
must recognize when a crate's documentation is in the same output directory. The
current threshold for "local documentation for crate X being available" is
whether the directory "doc/X" exists.

This change modifies the build system to have new dependencies for each
directory of upstream crates for a rustdoc invocation. This will ensure that
when building documentation that all the crates in the standard distribution are
guaranteed to have relative links to one another.

This change is prompted by guaranteeing that offline docs always work with one
another. Before this change, races could mean that some docs were built before
others, and hence may have http links when relative links would suffice.

Closes #14747

Apparently one of the linux bots doesn't have the USER variable defined, and
this fix will likely land more quickly than a fix to the bots.