itx.rs: Replace C types with Rust equivalents

[randomPoison]

• Last bit of trivial cleanup for Cleanup c2rust output in itx.rs #817.

[kkysen]

LGTM but I'm not sure if it's too important to do this. That said, if we are, it makes sense to replace libc::ptrdiff_t with isize, too, I think.

[randomPoison]

Ah good catch I missed that we also had ptrdiff_t in there. Cleaned that up as well. And yeah, this isn't priority work but since it takes literally seconds to do I figured I'd quickly do this as part of closing out the c2rust cleanup issues.