Mutability of variables captured by procs is ignored

``` rust
fn main() {
    let mut x = 1;
    proc() { x = 2 };
}
```

```
proc-mut.rs:3:14: 3:15 error: cannot assign to immutable captured outer variable in a heap closure
proc-mut.rs:3     proc() { x = 2 };
                           ^
```
