Rollup merge of #111009 - scottmcm:ascii-char, r=BurntSushi
Add `ascii::Char` (ACP#179)
ACP second: https://github.com/rust-lang/libs-team/issues/179#issuecomment-1527900570
New tracking issue: https://github.com/rust-lang/rust/issues/110998
For now this is an `enum` as `@kupiakos` [suggested](https://github.com/rust-lang/libs-team/issues/179#issuecomment-1527959724), with the variants under a different feature flag.
There's lots more things that could be added here, and place for further doc updates, but this seems like a plausible starting point PR.
I've gone through and put an `as_ascii` next to every `is_ascii`: on `u8`, `char`, `[u8]`, and `str`.
As a demonstration, made a commit updating some formatting code to use this: https://github.com/scottmcm/rust/commit/ascii-char-in-fmt (I don't want to include that in this PR, though, because that brings in perf questions that don't exist if this is just adding new unstable APIs.)
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