Add test module, def AVL create()/free().

data_structures/avl_tree/src/avl_tree.(c, h)
	-Define `AVLTree_create()` and `AVLTree_free()`.
	-Change `AVL` function/struct prefix to `AVLTree`.

data_structures/avl_tree/src/test.c
	-Move compiled executable's entry point to a unit-testing
	module.