libstd: Implement `StrBuf`, a new string buffer type like `Vec`, and port all code over to use it.
Rebased & tests-fixed version of https://github.com/mozilla/rust/pull/13269
The manual, tutorial and guides need the feature gates quite often,
unfortunately, so this is the low-cost path to migrating to use
rustdoc. This is only activated for pure-Markdown files.
Preferably this would be avoided: #12773
markdown files.
This means that
# Foo
## Bar
# Baz
### Qux
## Quz
Gets a TOC like
1 Foo
1.1 Bar
2 Baz
2.0.1 Qux
2.1 Quz
This functionality is only used when rendering a single markdown file,
never on an individual module, although it could very feasibly be
extended to allow modules to opt-in to a table of contents (std::fmt
comes to mind).
This theoretically gives rustdoc the ability to render our guides,
tutorial and manual (not in practice, since the files themselves need to
be adjusted slightly to use Sundown-compatible functionality).
Fixes#11392.
