Sparsify bitmap lookups

[cmdevries]

Sparsifying storage of bitmap lookups can make them more efficient.

See https://github.com/cmdevries/LMW-tree/blob/master/src/lmw/BitMapList8.h#L53 and https://github.com/cmdevries/LMW-tree/blob/master/src/lmw/BitMapList16.h#L55.

Also can make it a single class while we are at it.