Revisit the exercise about the Iterator pattern.  Provide a facade that
combines the tree class and its iterator.  Keep in mind that there
could exist additional iterators for additional traversal policies.

The implementation has characteristics of Delegation (the class
delegates most operations to class BinaryTree), Adapter (the class
modifies the signature of some methods of class BinaryTree), and
Proxy (the class performs similarly to BinaryTree, but hides some
of its operations).