Implemented simple show method inside Dequeue to hide unnecessary (large!) implementation detail from the client, in particular when using REPL
Fixed bugs related to Dequeue functions front and back. Front would give garbage data when called on a newly created queue, and back when popping a queue from the front. I've added additional tests to ensure consistency throughout. This would also produce erroneous behavior with Stack's top operation, which would give garbage data on a newly created stack.