overlapping instances should be allowed for traits with no methods

This is what will make inherited kinds work:

```
trait Bar : Copy {
    fn g();
}

struct A {
    x: int
}

impl A : Bar {
    fn g() { }
}

fn main() {
}
```

The user is implementing `Copy`, but the compiler also implements `Copy` implicitly.

This will also allow kind-based 'assertions':

```
struct Foo { ... }

// Overlapping instance guarantees that `Foo` is copyable
impl Foo: Copy;
```
