Rollup merge of #137432 - djscythe:char_u8_str_as_ascii_unchecked, r=scottmcm

Add as_ascii_unchecked() methods to char, u8, and str

This PR adds the `as_ascii_unchecked()` method to `char`, `u8`, and `str`, allowing users to convert these types to `ascii::Char`s (see #110998) in an `unsafe` context without first checking for validity. This method was already available for `[u8]`, so this PR makes the API more consistent across other types.

Author: matthiaskrgr