structures: fix memory leak in red-black tree

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
red-black tree: make sure that rule 2 is respected

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
red-black tree: implement insert cases 1 and 2

Link: http://en.wikipedia.org/wiki/Red%E2%80%93black_tree
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>