Add a new tuplestore API function, tuplestore_putvalues(). This is

Add a new tuplestore API function, tuplestore_putvalues(). This is
identical to tuplestore_puttuple(), except it operates on arrays of
Datums + nulls rather than a fully-formed HeapTuple. In several places
that use the tuplestore API, this means we can avoid creating a
HeapTuple altogether, saving a copy.