Auto merge of #25028 - bluss:drain-string, r=alexcrichton

Auto merge of #25028 - bluss:drain-string, r=alexcrichton

collections: Implement String::drain(range) according to RFC 574

`.drain(range)` is unstable and under feature(collections_drain).

This adds a safe way to remove any range of a String as efficiently as
possible.

As noted in the code, this drain iterator has none of the memory safety
issues of the vector version.

RFC tracking issue is #23055