libnum: Make RandBigInt trait public

Closes #12383.
rustc: remove some unnecessary `transmute`s.

These can all be done by implicit or explicit &T -> *T casts, which are
more restricted and so are safer.
Fix getting/setting huge env vars on windows

On windows, the GetEnvironmentVariable function will return the necessary buffer
size if the buffer provided was too small. This case previously fell through the
checks inside of fill_utf16_buf_and_decode, tripping an assertion in the `slice`
method.

This adds an extra case for when the return value is >= the buffer size, in
which case we assume the return value as the new buffer size and try again.

Closes #12376
debuginfo: Add support for simd types
debuginfo: Move test case from run-pass to debug-info directory so it does not interfere with RUSTFLAGS=-g
debuginfo: Fix a RUSTFLAGS incompatibility in test runner.
debuginfo: Re-enable test case that should have worked all along.
Tweak how preference factors into linkage

The new methodology can be found in the re-worded comment, but the gist of it is
that -C prefer-dynamic doesn't turn off static linkage. The error messages
should also be a little more sane now.

Closes #12133
rustc: support dumping the AST as JSON

This is mostly useful for working on rustc, when one is unfamiliar with the
AST a particular construct will produce. It's a -Z flag as it's very much for
debugging.

Closes #10485
Don't recurse twice into ExprParen in vtable checking.
Replaced method_map_entry with method_origin and cleaned up vtable checking a bit.
adjust to currently used style
Resolve the vtables for method calls to generic Drop impls with trait bounds.
auto merge of #12387 : cmr/rust/ast-json, r=alexcrichton

See the commits
librustc: Remove unique vector patterns from the language.

Preparatory work for removing unique vectors from the language, which is
itself preparatory work for dynamically sized types.
Fix sending/try_recv on channels off the runtime

The fairness yield mistakenly called `Local::take()` which meant that it would
only work if a local task was available. In theory sending on a channel (or
calling try_recv) requires no runtime because it never blocks, so there's no
reason it shouldn't support such a use case.

Closes #12391
auto merge of #12394 : huonw/rust/transmute--, r=pnkfelix

These can all be done by implicit or explicit &T -> *T casts, which are
more restricted and so are safer.
auto merge of #12244 : pcwalton/rust/deuniquevectorpatterns, r=pcwalton

Preparatory work for removing unique vectors from the language, which is
itself preparatory work for dynamically sized types.

r? @brson
auto merge of #12392 : aepsil0n/rust/fix/pub_randbigint, r=huonw

Closes #12383.

Test suite did not capture this and can't as long as it is in the same module hierarchy. This is probably something that should be addressed in the future.
move extra::test to libtest