rustc_arena: add `alloc_str` Two places called `from_utf8_unchecked` for strings from `alloc_slice`, and one's SAFETY comment said this was for lack of `alloc_str` -- so let's just add that instead!
rustc_arena: add `alloc_str` Two places called `from_utf8_unchecked` for strings from `alloc_slice`, and one's SAFETY comment said this was for lack of `alloc_str` -- so let's just add that instead!