libsyntax: remove dead code find_linkage_metas

Closes #14329
Add a b'x' byte literal of type u8.
Add a b"xx" byte string literal of type &'static [u8].
Add br##"xx"## raw byte string literals.
Document the byte, byte string, and raw byte string literals.
Refactor backslash-escape parsing to share similar code.

Move into a new syntax::parse::lexer::StringReader method the code
that was almost duplicated for parsing backslash-escapes in
byte, byte string, char, and string literals.
Fix some violations of stronger guarantees for mutable borrows.

See 159e27aebb940926ccf1bad0b2b12087d36ad903
Remove TraitStore from ty_trait

Use ty_rptr/ty_uniq(ty_trait) rather than TraitStore to represent trait types.
Also addresses (but doesn't close) #12470.
Part of the work towards DST (#12938).

[breaking-change] lifetime parameters in `&mut trait` are now invariant. They used to be contravariant.
auto merge of #14956 : nathantypanski/rust/master, r=alexcrichton

Closes #14329

Recent-ish uses of `find_linkage_metas` from my `git grep` output are [here](https://gist.github.com/nathantypanski/b9d2d453718a22765f5c), for those interested who wish to tread through the history.
Fix expected error message in a test.

The change is a result of the char/string parsing refactor.
auto merge of #14869 : nick29581/rust/tstore, r=nmatsakis

Use ty_rptr/ty_uniq(ty_trait) rather than TraitStore to represent trait types.
Also addresses (but doesn't close) #12470.
Part of the work towards DST (#12938).
auto merge of #14880 : SimonSapin/rust/byte-literals, r=alexcrichton

See #14646 (tracking issue) and rust-lang/rfcs#69.

This does not close the tracking issue, as the `bytes!()` macro still needs to be removed. It will be later, after a snapshot is made with the changes in this PR, so that the new syntax can be used when bootstrapping the compiler.
Revamp TaskBuilder API

This patch consolidates and cleans up the task spawning APIs:

* Removes the problematic `future_result` method from `std::task::TaskBuilder`,
  and adds a `try_future` that both spawns the task and returns a future
  representing its eventual result (or failure).

* Removes the public `opts` field from `TaskBuilder`, instead adding appropriate
  builder methods to configure the task.

* Adds extension traits to libgreen and libnative that add methods to
  `TaskBuilder` for spawning the task as a green or native thread.

Previously, there was no way to benefit from the `TaskBuilder` functionality and
also set the scheduler to spawn within.

With this change, all task spawning scenarios are supported through the
`TaskBuilder` interface.

Closes #3725.

[breaking-change]
Fallout from TaskBuilder changes

This commit brings code downstream of libstd up to date with the new
TaskBuilder API.