pg_dump: provide a stable sort order for rules.

pg_dump: provide a stable sort order for rules.

Previously, we sorted rules by schema name and then rule name;
if that wasn't unique, we sorted by rule OID.  This can be
problematic for comparing dumps from databases with different
histories, especially since certain rule names like "_RETURN"
are very common.  Let's make the sort key schema name, rule name,
table name, which should be unique.  (This is the same behavior
we've long used for triggers and RLS policies.)

Andreas Karlsson

Discussion: https://postgr.es/m/b4e468d8-0cd6-42e6-ac8a-1d6afa6e0cf1@proxel.se