[RDY] Remove project-specific integer types: short_u, int_u, long_i.

This is the first of a series of PR's aiming to remove project-specific typedefs for integer types, as requested in #459.

This PR deals with `short_u`, `int_u`, and `long_i`. See commit messages for details.

I'm now working on `long_u`, but that will take a while, and I'll open another PR for that.