Macros can not parse and expand to associated types

``` rust
macro_rules! items {
    () => {
        type A = ();
        fn a() {}
    }
}

trait Foo {
    type A;
    fn a();
}

impl Foo for () {
    items!();
}

fn main() {

}
```

```
<anon>:3:9: 3:13 error: expected one of `extern`, `fn`, or `unsafe`, found `type`
<anon>:3         type A = ();
                 ^~~~
```
