AK: Add CircularDeque.

This class inherits from CircularQueue and adds the ability dequeue
from the end of the queue using dequeue_end().

Note that I had to make some of CircularQueue's fields protected to
properly implement dequeue_end.

AK/CircularQueue.h

@@ -77,7 +77,7 @@ public:
 
     int head_index() const { return m_head; }
 
-private:
+protected:
     friend class ConstIterator;
     T m_elements[Capacity];
     int m_size { 0 };