Rename W_BaseIterator and subclasses

* A W_BaseIterator instance is not a wrapped object, so should not have
  the w_ prefix.
* Prevent confusion with app-level iterator objects in spl.
Make interp-level iterators follow app-level Iterator interface more closely
more Iterator interface
yet more Iterator interface
Give ArrayIterator object its own interp-level iterator
Don't crash if the item is deleted during iteration
fix translation