Expose arithmetic operations for POSIX arithmetic types

[yallop]

No description provided.

[mcclurmc]

I was about to raise a feature request for this, but I saw that you've already raised this, so I'll just add my +1. I'd like to be able to use types like uid_t and gid_t in OCaml code, but right now those types are opaque and I can't do anything with them. Functions to convert to int, int32, etc., would be useful.