for-loop-bound vars not hygienic

This should signal an error:

``` rust
#![feature(macro_rules)]

macro_rules! f(() => (n))

fn main() -> (){
    for n in range(0, 1) {
        println!("{}", f!());
    }
}
```

I've got a fix for this, I'm just adding this so that I have an issue number to attach to my test case.
