ENH: Add PackedBoolList class (previously just a typedef)

ENH: adjust internal storage of PackedList to ensure that all unused
     internal storage elements are guaranteed to always be bit-wise
     zero. This makes the PackedBoolList implementation easier, and
     also greatly eases much of the book-keeping.
     Bits are now also zeroed on PackedList::clear()

Note: in PackedList, require packing at least 2 items otherwise it is
     more efficient to use a normal list.