pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e57519a4637a8d88ae993ac1273d2b59d03a0f75

Modified Files
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src/backend/lib/rbtree.c | 62 ++++++++++++++++++
src/include/lib/rbtree.h | 2 +
src/test/modules/test_rbtree/test_rbtree.c | 102 +++++++++++++++++++++++++++++
3 files changed, 166 insertions(+)