* Breaking change: Make StableMinHeap/StableMaxHeap stop inheriting from SplHeap to be more memory efficient.
* Properly sort in StableSortedListSet::__construct and __set_state
* Deduplicate code.
* Reduce size/capacity limits to the same limits as array for Deque.
* Add ImmutableSequence::map(), filter()
* Fix bug in Deque::contains(), Deque::indexOf()