redefine superclass in Object as always returning a class

[HoneyryderChuck]

the only instance of a nil superclass is basic object.

This is a proposal to make this definition more precise. open for discussion.

[soutaro]

@HoneyryderChuck

I don't think moving the superclass method definition to Object is a good idea. The method is defined in Class class.

Having a special definition in BasicObject might make sense, because we can skip testing superclass value every time, except it's BasicObject.