Change suffixes to `isize` and `usize`

[nikomatsakis]

Tracking issue for rust-lang/rfcs#573

[pnkfelix]

Reopening, apparently us/is is still being silently accepted?

Example on playpen:

fn main() {
    let foo = 2us;
    println!("foo: {}", foo);
}

See also http://internals.rust-lang.org/t/how-did-the-is-us-suffixes-survive/1871

[pnkfelix]

Nominating: I want to figure out if a fix for this needs to be backported to beta, or to 1.0, or if we are stuck accepting it for now (or forever).

[nikomatsakis]

The problem seems to be in this commit: 54f16b8

[nikomatsakis]

Specifically here.