string_extract_if: initial implementation

[GrigorenkoPV]

Tracking issue: #154318

[rustbot]

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[Mark-Simulacrum]

Since we only ever remove a full char, how can we break the UTF-8 property? AFAICT, the Vec should always be initialized and a valid String. The removal operations this performs should correspond directly to calling String::drain(start..end) after a sequence of false returns. That is a tiny bit less efficient since it'll re-check the start/end characters are at a UTF-8 boundary, but that check is O(1) so it shouldn't be that much slower.

I think the current implementation doesn't actually optimize to copy larger chunks only when needed, it looks like we copy each char (likely via call to memmove) which seems likely to be pretty inefficient?