Implemented a count leaves and a for each leaf function

That are some weird strings in our test cases :/

Signed-off-by: Toon Schoenmakers <nighteyes1993@gmail.com>
Implemented the teardown functions

Yes it is really called bst_dig_up_tree, I'm going with a theme
here you know.

Signed-off-by: Toon Schoenmakers <nighteyes1993@gmail.com>