Various improvements

1) By writing the Count() function in C, we speed things up by about 50%. It should actually be a lot more than 50%, but I can't convince cgo to use SSE2 at the moment. In any case, 50% is significant enough.

2) I fixed a bug in NextSet.

3) I have added cardinality functions. Given two bitset, you can now compute the cardinality (Count) of their intersections without having to materialize the answer. (Great potential speed boost.)