Add unit test for traversals.

data_structures/binary_tree/src/test.c
	-Define `test_traversals()`, to test `BinaryTree_preOrder()`,
	`BinaryTree_inOrder()`, and `BinaryTree_postOrder()`.
Simplify traversal implementations.

data_structures/binary_tree/src/binary_tree.c
	-Simplify some unnecessary control-flow in
	`BinaryTree_preOrder()`, `BinaryTree_inOrder()`, and
	`BinaryTree_postOrder()`.
Define BinaryTree_travBreadth().

data_structures/binary_tree/src/binary_tree.(c, h)
	-Implement a breadth-first traversal.