We would incref them before putting them into the args array.
Now we keep some extra information during the rewriting process, which
lets rearrangeArguments say whether any of the values need to
be decreffed.  It's pretty rare that they need to so I think this
should be some nice savings (20% on a specifically-targeted microbenchmark,
no change on raytrace.py).

I'm a little bit worried though that since it is so rare to need to do the
decref, it would be easy to forget to check for it.

Well I think we already were forgetting it in some places... but at least now
it's an explicit arg you have to manage so maybe it's less likely to cause mistakes.