PackedList improvements

- dropped auto-vivification for now (performance issue), but reworked to
  allow easy reinstatement
- derived both iterator and const_iterator from iteratorBase and use
  iteratorBase as our proxy for non-const access to the list elements.
  This allows properly chaining assignments:
     list[1] = list[2];
     list[1] = list[2] = 10;
- assigning iterators from iteratorBase or other iterators works:
     iterator iter = list[20];
- made template parameter nBits=1 the default