Allow to use HeapTupleData embedded in [Buffer]HeapTupleTableSlot.

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