Driver compiler doesn't handle extended classes

[AfterLifeLochie]

The driver compiler doesn't support extended classes at all. Instead, it considers only the interfaces on the source and the destination and ignores any superclasses.

It would be useful for the merger engine to support importing extensions from source classes into destination classes; this would essentially mean asserting that there is only one superclass not Object between source and destination, then ensuring that the object created in <init> is of the correct supertype (ie, either object for no supertype, or an instance of the superclass).