Speed up BitIndexIterator by unrolling

[Dandandan]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
BitIndexIterator is critical to performance of lots of kernels (filter / take and anything dealing with nulls).

This can be made faster

Describe the solution you'd like

Implement optimization described here:
https://lemire.me/blog/2019/05/03/really-fast-bitset-decoding-for-average-densities/

Describe alternatives you've considered

Additional context