@trait, ~trait, and &trait should be supported

These forms are not currently supported. Example:

```
trait T {
}

struct S {
    f : f32;
}

impl of T for S {
}

fn main() {
    let s = @S { f : 100.0f32 };
    let t = s as @T;
}
```

This does not currently work (the compiler will complain that the cast is non-scalar).
