`!` is not a valid ty token in macros

This program:

``` rust
macro_rules! foo(($t:ty) => ()) 

foo!(!)                         

fn main() {}                    
```

fails with

```
foo.rs:3:5: 3:6 error: expected type, found token NOT
foo.rs:3 foo!(!)
              ^
```

I ran into this when creating a condition which returned `!` (not sure if that's even the right use of conditions...)
