ALTER TABLE: skip FK validation when it's safe to do so

We already skip rewriting the table in these cases, but we still force a
whole table scan to validate the data.  This can be skipped, and thus
we can make the whole ALTER TABLE operation just do some catalog touches
instead of scanning the table, when these two conditions hold:

(a) Old and new pg_constraint.conpfeqop match exactly.  This is actually
stronger than needed; we could loosen things by way of operator
families, but it'd require a lot more effort.

(b) The functions, if any, implementing a cast from the foreign type to
the primary opcintype are the same.  For this purpose, we can consider a
binary coercion equivalent to an exact type match.  When the opcintype
is polymorphic, require that the old and new foreign types match
exactly.  (Since ri_triggers.c does use the executor, the stronger check
for polymorphic types is no mere future-proofing.  However, no core type
exercises its necessity.)

Author: Noah Misch

Committer's note: catalog version bumped due to change of the Constraint
node.  I can't actually find any way to have such a node in a stored
rule, but given that we have "out" support for them, better be safe.