Teach typechecker that uint is <plusable>.

[froystig]

At the moment,

fn main() {
  let uint i = uint(1);
  i += uint(1);
}

fails to compile, giving the error:

uintplus.rs:3:2 - 4:0:E:Error: mismatched types: uint vs. <plusable>

Fix this.

[froystig]

Teach the typechecker that uints are integral types. Closed by 66d313d.