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.)

library/core/src/ascii.rs

@@ -14,6 +14,10 @@ use crate::fmt;
 use crate::iter::FusedIterator;
 use crate::num::NonZeroUsize;
 
+mod ascii_char;
+#[unstable(feature = "ascii_char", issue = "110998")]
+pub use ascii_char::AsciiChar as Char;
+
 /// An iterator over the escaped version of a byte.
 ///
 /// This `struct` is created by the [`escape_default`] function. See its