PackedList - activated lazy evaluation
- moving back to original flat addressing in iterators means there is no performance issue with using lazy evaluation - set() method now has ~0 for a default value. We can thus simply write 'set(i) to trun on all of the bits. This means we can use it just like labelHashSet::set(i) - added flip() method for inverting bits. I don't know where we might need it, but the STL has it so we might as well too.