Support Inheritance

[jwhpryor]

Currently, class definitions must re-declare all of their parent classes interface if they want to be used, which is repetitive.

Also, subclasses must static_cast themselves to jobjects to be used in a context their subclass is required which loses type safety.

enum classes are basically syntactic sugar for a subclassing, so this likely needs to be implemented first.

It will be nice to unify the type system to enable interface of Object throughout Java. That said, Kotlin's Any is not Java's Object, so any solution should not presume this.