postgres-lambda-diff

try to push restrictions on the view down into the view subquery,
so that they can become indexscan quals or what-have-you rather than
being applied at the top level of the subquery.  7.0 and before were
able to do this, though in a much klugier way, and I'd hate to have
anyone complaining that 7.1 is stupider than 7.0 ...