Port to clap-rs

I did a number of things here:

the *add, *mod, and *del utilities, as well as id recieved substantial changes, probably what you'd call a rewrite. This was of course using clap-rs. These should all be very stable at this point (except unimplemented features in usermod, grr)

getty, passwd, and su are simply ported, using clap for arg parsing. I didn't change any of the logic, just swapped ArgParser for clap.

Fixed a bug with passwd and unset passwords

I'll open issues about the ones I didn't mention.

Solves #29 (closed)