Nullary enum variants shouldn't be able to be matched/declared with trailing parens ()

``` rust
enum Foo { Bar(), Baz() }
fn main() {
    println!("{}", match Bar { Bar() => 1, Baz() => 2 })
}
```

prints `1`.

This seems a little peculiar: I would certainly prefer that they match unit structs and require that they don't have the `()`.
