Implement the "Class System Extension" proposal
- Class and instance declarations would allow method implementations to be given for any methods in the class or any ancestor class.
- Whenever an instance declaration is visible there would always be a full set of instance declarations for all ancestor classes, by supplementing the set of explicitly given instance declarations that are visible in a module by automatically generated implicit instance declarations.
- The most specific method implementation would always be chosen (ie prefer an explicit instance method over a class method and prefer a subclass method to a superclass method)
- Modules would only export explicit instance declarations