Introduce `String::remove_matches` to remove all matches of a pattern in a string.

[frewsxcv]

Like replace(..., ""), but doesn't result in any allocations.

I started implementing this in #50015 but got stuck, mainly because of limitations in the Pattern APIs.

let mut s = String::from("Trees are not green, the sky is not blue.");
s.remove_matches("not ");
assert_eq!("Trees are green, the sky is blue.", s);

This issue has been assigned to @jcotton42 via this comment.

[jcotton42]

@rustbot claim