Rollup merge of #103919 - nnethercote:unescaping-cleanups, r=matklad

Unescaping cleanups

Some code improvements, and some error message improvements.

Best reviewed one commit at a time.

r? ````@matklad````

diff --cc compiler/rustc_lexer/src/lib.rs
Simple merge

diff --cc compiler/rustc_parse/src/lexer/mod.rs
Simple merge