Allow to use HeapTupleData embedded in [Buffer]HeapTupleTableSlot.

That avoids having to care about the lifetime of the
HeapTupleHeaderData passed to ExecStore[Buffer]HeapTuple(). That
doesn't make a huge difference for a plain HeapTupleTableSlot, but for
BufferHeapTupleTableSlot it can be a significant advantage, avoiding
the need to materialize slots where it's inconvenient to provide a
HeapTupleData with appropriate lifetime to point to the on-disk tuple.

It's quite possible that we'll want to add support functions for
constructing HeapTuples using that embedded HeapTupleData, but for now
callers do so themselves.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

src/include/executor/tuptable.h

@@ -16,6 +16,7 @@
 
 #include "access/htup.h"
 #include "access/tupdesc.h"
+#include "access/htup_details.h"
 #include "storage/buf.h"
 
 /*----------
@@ -246,6 +247,7 @@ typedef struct HeapTupleTableSlot
 	HeapTuple	tuple;		/* physical tuple */
 #define FIELDNO_HEAPTUPLETABLESLOT_OFF 2
 	uint32		off;		/* saved state for slot_deform_heap_tuple */
+	HeapTupleData tupdata;	/* optional workspace for storing tuple */
 } HeapTupleTableSlot;
 
 /* heap tuple residing in a buffer */