Modify tuptoaster's API so that it does not try to modify the passed

Modify tuptoaster's API so that it does not try to modify the passed
tuple in-place, but instead passes back an all-new tuple structure if
any changes are needed.  This is a much cleaner and more robust solution
for the bug discovered by Alexey Beschiokov; accordingly, revert the
quick hack I installed yesterday.
With this change, HeapTupleData.t_datamcxt is no longer needed; will
remove it in a separate commit in HEAD only.