Pretty printer doesn't handle `& &` correctly

``` rust
fn main() { & &"foo"; }
```

```
> rustc andand.rs --pretty
fn main() { &&"foo"; }
> rustc andand.rs --pretty | rustc -
<anon>:1:12: 1:14 error: unexpected token: `&&`
<anon>:1 fn main() { &&"foo"; }
                     ^~
```
