Module queues

Helper queues for use in graph searching.

• LIFO: Last in first out queue (stack)
• FIFO: First in first out queue
• Priority(fcn): Priority queue with items are sorted by fcn
• Random: Random queue
• q.append(item) -- add an item to the queue
• q.extend(items) -- equivalent to: for item in items: q.append(item)
• q.pop() -- return the top item from the queue
• len(q) -- number of items in q (also q.__len())