Properly mark mergeable/hashable equality operators (found by opr_sanity

testing).  Combine the formerly independent opclasses for the various
ISN types into opfamilies.  The latter causes some extra bleating from
opr_sanity, since the module doesn't provide complete sets of cross-type
operators, but it's still a good idea because it will give the planner
more information to work with.  The missing cross-type operators no longer
pose a risk of unexpected planner errors in 8.3, so there's no need to
insist on filling them in (and I gather it wouldn't be very sound
semantically to add them all).