Add sortsupport for gist_btree opclasses, for faster index builds.
Commit
16fa9b2b30 introduced a faster way to build GiST indexes, by
sorting all the data. This commit adds the sortsupport functions needed
to make use of that feature for btree_gist.
Author: Andrey Borodin
Discussion: https://www.postgresql.org/message-id/
2F3F7265-0D22-44DB-AD71-
8554C743D943@yandex-team.ru