Implementing deque

Deques (pronounced decks) are list-like containers that have fast appending and pop values from either end. The name deque comes from this action: double-ended queues. Deques are thread safe, meaning data is manipulated in such a way that all threads behave without overwriting data or otherwise operate with unintended actions. Popped values from either end of the deque have the same performance, regardless of being on the front or the rear of the queue.

For those familiar with big O notation, performance is O(1) for both front- and rear-popped values. For those unfamiliar with big O notation, this just means it takes the same amount of time for a deque to pop a value from the front as it does for the back. This is significant because lists, which have similar operations as the deque, are optimized for quick, fixed-length operations and take a performance hit of O(n) to memory movement when popping and inserting values, as they both modify the size and position of the data...