rust-clippy: add `is_empty` method to please the `clippy` gods I can see it being useful anyway.

rust: don't use a reference to a `Cow` This was caught by `clippy`.

rust-clippy: reassure `clippy` that 8 arguments is expected This API is a bit verbose, but refactoring it into a struct isn't better in this particular case IMO, and there is a lot of info to pass.

rust-clippy: use `write_all` since we're not expecting a partial write Clippy caught this. This is a good lint in general, but here this was not really a bug in this example code.

rust-clippy: tell clippy we want to keep those clauses separate This makes the cases more explicit, which is wanted in this specific instance.

rust-clippy: tell clippy we care about keeping those `if` clauses separate Thses were written this way because it spells out the logic in a more explicit manner.

rust-clippy: factor an `if` with the same duplicated clause Unlike the next changeset, this if is not really made clearer by keeping the cases separate.

rust: fix broken debug assertions These were caught by `clippy`. It appears no one uses the debug builds, which is how this wasn't caught before.

rust-narrow: fix loop that never loops This was caught by `clippy`. I guess the narrow tests leave something to be desired, since this previously only checked the first valid pattern.

rust: run `cargo clippy` These automatic fixes are good to have because they make the code more idiomatic and less surprising. The transform from `sort` -> `sort_unstable` is questionable, but this is only in a test, so it doesn't matter in our case.