* Backwards incompatible change: Change `Deque` APIs to be consistent with SplDoublyLinkedList and `array_push`: change pushBack/popBack/pushFront/popFront to push/pop/unshift/shift
* Backwards incompatible change: Remove `$preserve_keys` flag from `Vector::__construct` (Always reindex keys in order of iteration instead).
* Add isEmpty() method to datastructures
* Make exceeding the capacity limit for a vector a fatal error.
* Make Deque::push() and unshift() variadic.