Refactor List.__next__

To avoid stuff like #135 from happening, make it more transparent what's
going on in `List.__next__` by
- replacing the check for `full=True` with
  `isinstance(self, GeneratorList)`
- replacing the recursive call to `List.__next__` with
  `item = six.next(self._iter)`. This means one line of code is
  duplicated, but I think it's pays of in term of readability.