run-tests: refactor the HGPORT handling code to use a constant for the max val Simple enough, and then we don't need comments to cross-reference other areas of code that need to be kept in sync. Since the `_getreplacements()` list is now partially dynamically generated and needed to change anyway, also move the one line that was being appended separately to the list for some reason, for clarity.

rust: fix darwin build

rust: remove `atty` dependency It is fully replaced with the now stable `std::io::IsTerminal` trait. This was the last dependency flagged as a warning by `cargo audit`, aside from `cpython` which we know about all too well: the plan is to transition to PyO3 soon-ish.

rust: update `env_logger` dependency Aside from being more up-to-date in general, this makes its own dependency on the unmaintained and now useless `atty` disappear.

rust: update `clap` to the latest 4.x version This brings in more up-to-date dependencies, some bug fixes (none of which are relevant yet), and slightly improved compile times.

rust: fix `cargo doc` warnings It makes sense to keep our doc build happy, even if it is lacking.

rust-compression: move the `Send` bound to the `Compressor` trait All compressors should be `Send`, might as well make any future use easier.