Add missing inequality searches to rbtree

Add missing inequality searches to rbtree

PostgreSQL contains the implementation of the red-black tree.  The red-black
tree is the ordered data structure, and one of its advantages is the ability
to do inequality searches.  This commit adds rbt_find_less() and
rbt_find_great() functions implementing these searches.  While these searches
aren't yet used in the core code, they might be useful for extensions.

Discussion: https://postgr.es/m/CAGRrpzYE8-7GCoaPjOiL9T_HY605MRax-2jgTtLq236uksZ1Sw%40mail.gmail.com
Author: Steve Chavez, Alexander Korotkov
Reviewed-by: Alexander Korotkov