Add cross-type comparisons to contrib/btree_gin.

Add cross-type comparisons to contrib/btree_gin.

Extend the infrastructure in btree_gin.c to permit cross-type
operators, and add the code to support them for the int2, int4,
and int8 opclasses.  (To keep this patch digestible, I left
the other datatypes for a separate patch.)  This improves the
usability of btree_gin indexes by allowing them to support the
same set of queries that a regular btree index does.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
Discussion: http://postgr.es/m/262624.1738460652@sss.pgh.pa.us