add support for inheriting from C++ base classes in the C API

[anderslanglands]

We'll need to use the same sort of infrastructure as for stdfunction wrapping, but:

• we'll need to generate trampolines for each virtual method in the base class we're inheriting from
• the user will need to pass a virtual function table in to the generated constructors

[anderslanglands]

Initial support for this is added in babble-0.6 but needs further testing