Silent overflow on debug beta/nightly

The standard wishlist

``` rust
fn main() {
    let mut it = 255u8..;
    println!("{:?}", it.next());
}
```

no longer panics, but AFAICT from the code this is not due to any deliberate change. The Iterator implementation is start.add_one()/swap, and add_one() is self + 1. This sounds like some overflow checks are being lost somehow.
