Allow VirtualFormalParameters to be extended on demand

[johannesduesing]

Currently, the VirtualFormalParameters are computed once and are then final. They depend on the number of DeclaredMethods that have a single defined method upon initialization. Since the set of declared methods can be extended on demand, this is not deterministic - we likely want to update the virtual formal parameters in a similar way.

This issue came up in this PR discussion

[johannesduesing]

This will likely be more complicated than expected. VirtualFormalParameters represent the receiver object for non-static methods. For virtual declared emthods, we do not know whether they are static or not. Thus, we might need to pass a "static-hint" when creating VirtualDeclaredMethod objects in order to correctly compute virtual formal parameters.