int/uint -> isize/usize

[pnkfelix]

int/uint -> isize/usize; i/u -> is/us

Authored by huon, some additional shepherding by pnkfelix.

[breaking-change]

[rust-highfive]

r? @brson

(rust_highfive has picked a reviewer for you, use r? to override)

[pnkfelix]

In my local make check run, it currently is up to rustc: x86_64-apple-darwin/stage2/test/rustctest-x86_64-apple-darwin

However, I have deliberately ignored a failing test: src/test/compile-fail/lifetime-inference-give-expl-lifetime-param-3.rs -- which seems to be a problem where a line break is being inserted by the pretty printer since the line has grown (with the int -> isize conversion), and the expected error output is not prepared for that line break.

[nikomatsakis]

/me takes over

[nikomatsakis]

See #20754