In commit b8d7f053, we needed to fix ExecEvalWholeRowVar to not change
the state of the slot it's copying.  The initial quick hack at that
required two rounds of tuple construction, which is not very nice.
To fix, add another primitive to tuptoaster.c that does precisely what
we need.  (I initially tried to do this by refactoring one of the
existing functions into two pieces; but it looked like that might hurt
performance for the existing case, and the amount of code that could
be shared is not very large, so I gave up on that.)

Discussion: https://postgr.es/m/26088.1490315792@sss.pgh.pa.us