Idea for minor tstore optimization

I notice that several of the call sites of tuplestore_puttuple() start
with arrays of datums and nulls, call heap_form_tuple(), and then switch
into the tstore's context and call tuplestore_puttuple(), which
deep-copies the HeapTuple into the tstore. ISTM it would be faster and
simpler to provide a tuplestore_putvalues(), which just takes the datum
+ nulls arrays and avoids the additional copy.

-Neil