HashTbl changes

- iterators store pointers instead of references to the HashTbl.
  This lets us use the default bitwise copy/assignment

- add empty constructor for iterators. It returns the equivalent to end().
  This lets us do this:
      HashTbl<label>::iterator iter;
      // some time later
      iter = find(Value);

- erase(const HashTbl<AnyType, Key, AnyHash>&) is now more generous.
  Only the Key type matters, not the hashing function.