mut qualifier cannot be applied to closure arguments

The following example should parse and work correctly:

```
fn swap(f: &fn(~[int]) -> ~[int]) -> ~[int] {
    let x = ~[1, 2, 3];
    f(x)
}

fn main() {
    let v = swap(|mut x| { x.push(4); x });
    let w = do swap |mut x| { x.push(4); x };
    assert_eq!(v, w);
}
```

It does not parse now (`mut` in identifier position)
