Auto merge of #31052 - bluss:split-at-mut-str, r=alexcrichton

Auto merge of #31052 - bluss:split-at-mut-str, r=alexcrichton

Use raw pointers to avoid aliasing in str::split_at_mut

Introduce private function  from_raw_parts_mut for str to factor out the logic.

We want to use raw pointers here instead of duplicating a &mut str, to
be on safer ground w.r.t rust aliasing rules.

This has already been fixed for slices in PR #27358, issue #27357