Allows moving in owned values into multiple closures

The following testcase crashes at runtime when call_f is called the second time. It appears rustc accepts to move the value t into "both" heap closures.

```
fn call_f(f: ~fn:Owned() -> int) -> int {
    f()
}

fn main() {
    let t = ~3;

    call_f(|| { *t + 1 });
    call_f(|| { *t + 1 });
}
```

Tested with version f1e09d6 / rustc 0.6 (f1e09d6 2013-06-26 23:07:41 -0700)

Assign to @bblum 
