Clean up various memory leaks within plpgsql, and re-enable the

Clean up various memory leaks within plpgsql, and re-enable the
exec_eval_simple_expr shortcut, which was diked out in 7.1 because it
leaked too much space.  CVS tip now leaks no memory in Chris Ruprecht's
example, which formerly leaked to the tune of 500 MB.  (Much of this
is work that Jan already did; this commit just cleans up around the
edges.)