constants are not constant due to ptr->int casts

``` rust
fn main() {
    const X: u32 = 1;
    const Y: uint = &X as *const u32 as uint;
    println!("{}", Y);
}
```

Fixing this requires backwards incompatible changes to constant expressions. At a minimum, casts from pointers to integers need to be forbidden. It's possible that there are other issues.
