rustdoc: make langstring parsing more robust

This changes the parsing of the language string
in code examples so that unrecognized examples
are not considered Rust code. This was, for example,
the case when a code example was marked `sh` for shell
code.

This relieves authors of having to mark those samples
as `notrust`.

Also adds recognition of the positive marker `rust`.

By default, unmarked examples are still considered rust.

If any rust-specific tags are seen, code is considered
rust unless marked as "notrust".

Adds test cases for the detection logic.
test: Enable #9205-related tests on windows

Fixes #9205.
auto merge of #14591 : klutzy/rust/issue-9205, r=thestinger

Fixes #9205.
rustdoc: Fix some more broken links
rustdoc: Filter private methods from inlined impls

Closes #14583
mk: Less noisy rustdoc invocations
rustdoc: Ensure external impls are inlined once

If the type associated with the impl is `pub use`'d or referenced twice in a
downstream crate, the impl will attempt to be inlined twice.

Closes #14584
auto merge of #14569 : skade/rust/rustdoc-robust-langstring-parsing, r=alexcrichton

This changes the parsing of the language string
in code examples so that unrecognized examples
are not considered Rust code. This was, for example,
the case when a code example was marked `sh` for shell
code.

This relieves authors of having to mark those samples
as `notrust`.

Also adds recognition of the positive marker `rust`.

By default, unmarked examples are still considered rust.
Fix deriving Encodable trait for unit structs

Closes #14021
auto merge of #14596 : Sawyer47/rust/encodable-fix, r=alexcrichton

Closes #14021
docs: Stop using `notrust`

Now that rustdoc understands proper language tags
as the code not being Rust, we can tag everything
properly.

This change tags examples in other languages by
their language. Plain notations are marked as `text`.
Console examples are marked as `console`.

Also fix markdown.rs to not highlight non-rust code.
doc: Remove use of `pub use` globs
syntax: Remove use of `pub use` globs

`quote_expr!` now injects two more (priv) `use` globs.
This may cause extra unused_imports warning.
rustdoc: Deduplicate lists of implementors

Inlining caused implementors to show up multiple times.

cc #14584
rustdoc: Correctly inline required/provided methods

Apparently relying on provided_source in ty::Method is unreliable!

Closes #14594
test: Add tests for closed issue #10764

Closes #10764
Improve code reuse in check_match::specialize()
Remove further code duplication
auto merge of #14509 : klutzy/rust/de-pub-use-glob, r=alexcrichton

This patchset removes `pub use` usage except for `test/`.
cc #11870
Document failure cases for `char_at` and friends.