std: Stabilize the `ascii` module

This commit performs a stabilization pass over the `std::ascii` module taking
the following actions:

* the module name is now stable
* `AsciiExt` is now stable after moving its type parameter to an `Owned`
  associated type
* `AsciiExt::is_ascii` is now stable
* `AsciiExt::to_ascii_uppercase` is now stable
* `AsciiExt::to_ascii_lowercase` is now stable
* `AsciiExt::eq_ignore_ascii_case` is now stable
* `AsciiExt::make_ascii_uppercase` is added to possibly replace
  `OwnedAsciiExt::into_ascii_uppercase` (similarly for lowercase variants).
* `escape_default` now returns an iterator and is stable
* `EscapeDefault` is now stable

Trait implementations are now also marked stable.

Primarily it is still unstable to *implement* the `AsciiExt` trait due to it
containing some unstable methods.

[breaking-change]

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