Set and print tree node keys instead of values.

Should extend to print both later.
Add binary search tree verification function.

Change make_tree()'s constant for indicating node absence to 0xDEAD to
be able to test trees with INT_MIN.
Add binary search tree.
Clean up list interface.

Now hides internal representation and uses a similar API to the other
data structures.
Rename min_heap_get() to min_heap_remove().

More consistent with other data structures.