Proxies can not inherit classes

[fabian-jung]

Proxies currently can not inherit other classes. The idea is to extend the template argument list by an additional parameter pack Base..., which will be public inherited from.

This will unlock mocking interfaces and safely proxy abstract classes.

Blocked by #49 . The generic way of implementing the proxy member function via an arg... variardic template falls apart for virtual inheritance. E.g. void T::foo(args...) will not overload a member function Base::foo(), because the argument list does not match. This leads to problems down the road.