BitvSet::is_disjoint is reversed

The implementation of `BitvSet::is_disjoint` is this:

``` rust
    #[inline]
    fn is_disjoint(&self, other: &BitvSet) -> bool {
        self.intersection(other).count() > 0
    }
```

It also shouldn't use `count`; it can just check whether the `intersection` iterator is empty directly.
