Type safe path file descriptors?

See this discussion over in nix: https://github.com/nix-rust/nix/pull/1863#discussion_r1040109997. I believe rustix can get the same treatment and it should be backwards compatible which is awesome. I'd like to push this through, but just wanted to double check that it'll be accepted before starting.

Start with the linked nix pull-request discussion and compare its proposed treatment with the corresponding rustix POSIX APIs. The issue names no files, tests, or concrete acceptance criteria, so confirm the intended backwards-compatible design with maintainers before implementation; done requires an accepted approach and its validation.

Written by the indexing model from the issue text.