Add newtypes for UserId, GroupId and ProcessId

[rnijveld]

We currently use type aliases, but that doesn't really add any type safety. Group ids, process ids and user ids can freely be interchanged because their definitions in libc are the same. If we use newtype wrappers (i.e. struct UserId(libc::uid_t) instead of type UserId = libc::uid_t), we can actually make a few more ensurances that they come from the right source. We should make construction of these newtypes very much explicit because of that (i.e. no From). That should prevent accidental conversion into one of the newtype definitions where it was not meant.

[pvdrz]

I'm stealing this

[van-sprundel]

I would like to work on this

UserId is used as const in timestamp tests, would it be okay to add lazy_static as a test dependency? Or add a const new method behind a test flag?

[squell]

About lazy_static; can't we achieve the same thing with the now-standard "once cell" features? rust-lang-nursery/lazy-static.rs#214 (I recall once having used it to provide backwards compatibility to rustc 1.65 during early sudo-rs development).

Our MSRV is currently 1.70 (and I think we can easily be persuaded to up it as far as 1.75), so once_cell is available.