implemented faster counting for set operations

[winstonewert]

For most of the operations, the implementation was straightforward. Simply have the iterators delegate count to BlockIter, then have BlockIter provide an optimized count() function that uses count_ones() to determine how many bits are set.

Intersection proved more problematic. The issue is that it is implemented using a Take wrapping the BlockIter. Take doesn't delegate count() to its internal iterator, so it will fall back to repeatedly invoking next() to count up the items. Take also doesn't provide a way to take back the wrapped iterator. What I did was re-implement the functionality of Take directly on the Intersection iterator.

I also considered creating a variant of TwoBitPosition which returned None when one of the subiterators ran out instead of padding with zeros. This would allow a similar, but not quite the same optimization. I elected to take the approach of reimplementing Take to minimize the impact of the change.

[pczarn]

Thanks! Rebased and merged.

[winstonewert]

Well... that's my longest code review process ever. :)

[pczarn]

@winstonewert One of mine was even longer. :)

By the way, we must have already met on StackExchange's Code Review forums.

[pczarn]

@winstonewert Not really longer... This one was 3 years and 5 months: https://github.com/tmuxinator/tmuxinator/pull/183