PackedList: encapsulate calculations in an iterator

- eliminated previous PackedBitRef class, the iterator does all of that and
  can also be used to (forward) traverse the list
- no const_iterator yet

- Note that PackedList is also a bit like DynamicList in terms of storage
  management and the append() method. Since the underlying storage in
  integer, any auto-vivified elements will also flood-fill the gaps with
  zero.