Restarg mutator

This is a simple mutator for restarg. It turns out the generic mutator does everything it needs.

I opted to move this out of the generic mutator so that we could continue to working on chipping away at these until the list of nodes it handles is empty. In the end, when everything is moved out, we can always move things back if we feel there are more nodes like this, that will probably never need any further customization.