Auto merge of #46735 - Manishearth:memchr-find, r=burntsushi
Use memchr for str::find(char)
This is a 10x improvement for searching for characters.
This also contains the patches from https://github.com/rust-lang/rust/pull/46713 . Feel free to land both separately or together.
cc @mystor @alexcrichton
r? @bluss
fixes #46693