[Rivet] Dangerous casting to FinalState

[David Grellscheid]

> For the following major release, it'd be good to solve this more
> completely. FinalStates are the only significant subset of projections
> which can have this problem, because FinalState itself is not virtual.

I don't understand what you mean here. Classes cannot be virtual / 
non-virtual, only methods can. A method is virtual if it has been marked 
so anywhere up the inheritance tree, it's not necessary to re-mark it as 
virtual in each child class.

   David