Add `os_str_bytes` feature

Adds some conversions from &str to &OsArgument as well as PartialEq<&str> (and vice versa), gated behind an unstable feature os_str_bytes.

Initial release

This has been sitting for four months because for a while we just didn't feel like reviewing the code that cuteness made. It looks and works fine on both Linux and Windows, and it's useful, so we're gonna publish it now~