Generalize hash and ordering support in amapi

Generalize hash and ordering support in amapi

Stop comparing access method OID values against HASH_AM_OID and
BTREE_AM_OID, and instead check the IndexAmRoutine for an index to see
if it advertises its ability to perform the necessary ordering,
hashing, or cross-type comparing functionality.  A field amcanorder
already existed, this uses it more widely.  Fields amcanhash and
amcancrosscompare are added for the other purposes.

Author: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com