sigils in front of macro_rules's expr nonterminal can convert a literal to a ~str

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

macro_rules! foo {
    ($e:expr) => { ~$e }
}

fn main() {
    *foo!(1); // fine
    *foo!("foo"); // error: type ~str cannot be derefenced
}
```

This would appear to go against Rust's non-textual macros.

(Writing `~($e)` in the macro works.)
