Unary `-` does not check for overflow

The following code prints `-2147483648 -2147483648`:

``` rust
fn main() {
    let i = 0x80000000u32 as i32;

    println!("{} {}", i, -i);
}
```

Both `0 - i` and `i / -1` generate an overflow error but it seems the unary `-` gladly wraps around.
