Add StorableBitVector
Parametrize Bitwise over Vector type

This only adds a type parameter to the Bitwise class. It does not
generalize any of the data structures.
Add PackedBits class to abstract over underlying bitvector representations
Rank9: Parametrize over bitvector type

This enables the bitvector to be backed by a Storable vector which can
be disk-backed with an `mmap` mapping.