parser rejects "ret ::f()"

```
fn f() { }
fn main() { ret ::f(); }
```

Is incorrectly rejected:

```
z.rs:2:16:2:18: error: expected ';' or '}' after expression but found ::
z.rs:2 fn main() { ret ::f(); }
                       ^~
```

fwiw, if I put parens around the call, it's accepted. But the pretty-printer takes out the parens, since they're obviously unnecessary ;)
