pgsql: A couple of marginal performance hacks for the information_schema

Log Message:
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A couple of marginal performance hacks for the information_schema views:
replace the old recursive-SQL-function implementation of _pg_keysequal()
with use of the built-in array containment operators, and change
table_constraints' UNION to UNION ALL.  Per discussion with Octavio Alvarez.

initdb not forced since this doesn't affect results, but you'd need to
initdb or reload the information_schema to see the new definitions.

Modified Files:
--------------
    pgsql/src/backend/catalog:
        information_schema.sql (r1.51 -> r1.52)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/information_schema.sql?r1=1.51&r2=1.52)