Implement some missing functions from AVL tree.  Factor out the insert(k, v) function into the base class.
1. Finish off the implementation of AVL trees.  Only one method missing: remove().
2. Share more code between Red-Black trees and AVL trees.
3. Remove some functions that can be implemented in terms of others.
Adjust return types for mapPartial() functions.
Start the work for the remove function -- skeleton is in place.