Add stratnum GiST support function

 Add stratnum GiST support function
 
 This is support function 12 for the GiST AM and translates
 "well-known" RT*StrategyNumber values into whatever strategy number is
 used by the opclass (since no particular numbers are actually
 required).  We will use this to support temporal PRIMARY
 KEY/UNIQUE/FOREIGN KEY/FOR PORTION OF functionality.
 
 This commit adds two implementations, one for internal GiST opclasses
 (just an identity function) and another for btree_gist opclasses.  It
 updates btree_gist from 1.7 to 1.8, adding the support function for
 all its opclasses.
 
 Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
 Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
 Reviewed-by: jian he <jian.universality@gmail.com>
 Discussion: https://www.postgresql.org/message-id/flat/CA+renyUApHgSZF9-nd-a0+OPGharLQLO=mDHcY4_qQ0+noCUVg@mail.gmail.com