- 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.