Auto merge of #111524 - scottmcm:escape-using-ascii, r=cuviper

`ascii::Char`-ify the escaping code in `core`

This means that `EscapeIterInner::as_str` no longer needs unsafe code, because the type system ensures the internal buffer is only ASCII, and thus valid UTF-8.

Come to think of it, this also gives it a (non-guaranteed) niche.

cc `@BurntSushi` as potentially interested
`ascii::Char` tracking issue: #110998

Author: bors