Redesign get_attstatsslot()/free_attstatsslot() for more safety and speed.

The mess cleaned up in commit da0759600 is clear evidence that it's a
bug hazard to expect the caller of get_attstatsslot()/free_attstatsslot()
to provide the correct type OID for the array elements in the slot.
Moreover, we weren't even getting any performance benefit from that,
since get_attstatsslot() was extracting the real type OID from the array
anyway.  So we ought to get rid of that requirement; indeed, it would
make more sense for get_attstatsslot() to pass back the type OID it found,
in case the caller isn't sure what to expect, which is likely in binary-
compatible-operator cases.

Another problem with the current implementation is that if the stats array
element type is pass-by-reference, we incur a palloc/memcpy/pfree cycle
for each element.  That seemed acceptable when the code was written because
we were targeting O(10) array sizes --- but these days, stats arrays are
almost always bigger than that, sometimes much bigger.  We can save a
significant number of cycles by doing one palloc/memcpy/pfree of the whole
array.  Indeed, in the now-probably-common case where the array is toasted,
that happens anyway so this method is basically free.  (Note: although the
catcache code will inline any out-of-line toasted values, it doesn't
decompress them.  At the other end of the size range, it doesn't expand
short-header datums either.  In either case, DatumGetArrayTypeP would have
to make a copy.  We do end up using an extra array copy step if the element
type is pass-by-value and the array length is neither small enough for a
short header nor large enough to have suffered compression.  But that
seems like a very acceptable price for winning in pass-by-ref cases.)

Hence, redesign to take these insights into account.  While at it,
convert to an API in which we fill a struct rather than passing a bunch
of pointers to individual output arguments.  That will make it less
painful if we ever want further expansion of what get_attstatsslot can
pass back.

It's certainly arguable that this is new development and not something to
push post-feature-freeze.  However, I view it as primarily bug-proofing
and therefore something that's better to have sooner not later.  Since
we aren't quite at beta phase yet, let's put it in.

Discussion: https://postgr.es/m/16364.1494520862@sss.pgh.pa.us

src/backend/tsearch/ts_selfuncs.c

@@ -163,28 +163,22 @@ tsquerysel(VariableStatData *vardata, Datum constval)
 	if (HeapTupleIsValid(vardata->statsTuple))
 	{
 		Form_pg_statistic stats;
-		Datum	   *values;
-		int			nvalues;
-		float4	   *numbers;
-		int			nnumbers;
+		AttStatsSlot sslot;
 
 		stats = (Form_pg_statistic) GETSTRUCT(vardata->statsTuple);
 
 		/* MCELEM will be an array of TEXT elements for a tsvector column */
-		if (get_attstatsslot(vardata->statsTuple,
-							 TEXTOID, -1,
+		if (get_attstatsslot(&sslot, vardata->statsTuple,
 							 STATISTIC_KIND_MCELEM, InvalidOid,
-							 NULL,
-							 &values, &nvalues,
-							 &numbers, &nnumbers))
+							 ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS))
 		{
 			/*
 			 * There is a most-common-elements slot for the tsvector Var, so
 			 * use that.
 			 */
-			selec = mcelem_tsquery_selec(query, values, nvalues,
-										 numbers, nnumbers);
-			free_attstatsslot(TEXTOID, values, nvalues, numbers, nnumbers);
+			selec = mcelem_tsquery_selec(query, sslot.values, sslot.nvalues,
+										 sslot.numbers, sslot.nnumbers);
+			free_attstatsslot(&sslot);
 		}
 		else
 		{