block arguments to macros are broken

```
#![feature(macro_rules)]

macro_rules! foo(
    ($b:block) => ($b)
)

fn main() {
    foo!({})
}
```

I don't see any reason why this shouldn't work.

Result:

```
huh.rs:4:21: 4:22 error: unexpected token: `an interpolated block`
huh.rs:4     ($b:block) => ($b)
```

rustc 0.11-pre-nightly (d35804e 2014-04-18 00:01:22 -0700)
