Warn when integer constant does not fit

[jruderman]

fn main()
{
    let x : int = 0x12345678_12345678_12345678;
    assert x != 0x12345678_12345678;
}

[catamorphism]

Nominating for milestone 5, production-ready

[catamorphism]

actually, I think #4220 covers this pretty well.