Destructors do not run when implemented on unit-like structs

``` rust
struct Bomb;

impl Drop for Bomb {
    fn finalize(&self) {
        println("Somebody set up us the bomb!");  // this is never printed
    }
}

fn main() {
    let b = Bomb;
    println(fmt!("b = %?", b));
}
```

Adding a single field like `{ a: int }` to the struct makes it print the expected message.
