Consistent varargs specialization in stagedfunctions (fix #8944)

This is basically an alternative version of @mlubin's fix. Anyone who is using stagedfunctions in their own code may want to test, but for me this appears likely to finally clear the path to merging the SubArray work. I'll wait until the Travis test clears, of course. (Sadly, `make testall` now force-reboots my laptop about 10% of the time, presumably due to memory-exhaustion, so I run it less and less. Thank goodness for Travis.)

While I would appreciate a code-review from Dr. Bezanson, Mr. Bezanson need not bother. Dr. Bezanson can clean up any messes I make, later.