Redefine AVLTree functions to use AVLTree_Node_t.

data_structures/avl_tree/src/avl_tree.c
	-Define and document `_createNode()`.
	-Redefine `AVLTree_insert()` and `_insert()` to wrap raw `data`
	in an `AVLTree_Node_t`, instead of passing it directly to the
	underlying `BinaryTree` functions.