Auto merge of #111222 - scottmcm:constify-is_ascii, r=thomcc

Constify `[u8]::is_ascii` (unstably)

UTF-8 checking in `const fn`-stabilized back in 1.63 (#97367), but apparently somehow ASCII checking was never const-ified, despite being simpler.

New constness-tracking issue for `is_ascii`: #111090

I noticed this working on `ascii::Char`: #110998

Author: bors