Auto merge of #130511 - bjoernager:const-char-encode-utf8, r=dtolnay

Support `char::encode_utf8` in const scenarios.

This PR implements [`rust-lang/rfcs#3696`](rust-lang/rfcs#3696).

This assumes [`const_slice_from_raw_parts_mut`](rust-lang/rust#67456).

Author: bors