Add String::replace_first and String::replace_last

[zachs18]

Rebase of #97977 (cc @WilliamVenner)

Convenience methods that use match_indices and replace_range to efficiently replace a substring in a string without reallocating, if capacity (and the implementation of Vec::splice) allows.

The intra-doc link to str::replacen is a direct url-based link to str::replacen in std's docs to work around #98941. This means that when building only alloc's docs (and not std's), it will be a broken link. There is precedent for this e.g. in core::hint::spin_loop which links to std::thread::yield_now using a url-based link and thus is a dead link when only building core's docs.

[rustbot]

r? @tgross35

rustbot has assigned @tgross35.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[tgross35]

I don't think I see any libs-api feedback on the other thread, and that is from a while back anyway. Could you file an ACP for this at https://github.com/rust-lang/libs-team/issues?

[tgross35]

These methods would be convenient but I feel like they should probably come with an in-place replace alternative.

[zachs18]

ACP opened: rust-lang/libs-team#506

@rustbot label S-waiting-on-ACP

I added String::replace_in_place (doing str::replace in-place) as a possible alternative/addition in the ACP, but I'll hold off on adding it if/until the ACP is approved with it.