A simple bitset stored in a single u64.

The downside of such a basic representation is, of course, that sets can only
store the numbers 0..64.  The major upside is that it's very compact and most
operations are fast.  The fact that it's `Copy` is also convenient.

If need to store elements from 0..64 in a set, `BitSet64` is probably the best
way to do it.