tuplestore_putvalues()

Attached is a patch that allows an array of Datums + nulls to be
inserted into a tuplestore without first creating a HeapTuple, per
recent suggestion on -hackers. This avoids making an unnecessary copy.
There isn't a really analogous optimization to be applied to tuplesort:
it takes either a TTS, an IndexTuple or a basic Datum, none of which
involve an extra copy.

BTW, I notice that almost all of the callers of the various
tuplestore_put methods switch into the tuplestore's context first. We
could simplify their lives a bit by having the tuplestore remember the
context in which it is allocated and do the switch itself. Perhaps it's
not worth bothering with at this point, though.

-Neil