i32::pow only handles overflow up to an exponent of 1023

Playpen: http://is.gd/zhwksu

``` rust
fn main() {
    let x: i32 = 2;

    println!("{}", x.pow(1024));
    println!("{}", x.pow(1023));
}
```

Prints:

```
0
thread '<main>' panicked at 'arithmetic operation overflowed', ../src/libcore/num/mod.rs:550
```

Interestingly, this only seems to happen with an even base. The overflow is detected correctly when `x` is set to 3 or other odd numbers.
