Add rbnode_comparer_t to common/rbtree/rbtree.h.
Add comparer to rbtree struct in common/rbtree/rbtree.h.
Change parameters of rbnode_comparer_t in common/rbtree/rbtree.h.
Rename rbnode_comparer_t to rbtree_comparer_t in common/rbtree/rbtree.h.