Superclasses for inline classes

[Brian Goetz]

> I kind of like your idea of a new flavor of constructor. If we drop the inference piece, it forces us to surface something in the language, but maybe it's subtle enough that, unlike a new flavor of class, it doesn't trip the "must include in Java 101" wire?

While I have little faith in textbook authors to correctly make this 
call (based on having reviewed dozens of textbooks), yes, this seems a 
reasonable balance of the concerns.