Changing the way how "companion class <-> companion" object links work

[DarkDimius]

Idea is to give every class that has a companion a method $companionObject that has term ref to a companion object as a return type. Every object that has a companion class will get a $companionClass method with a return type that indicates a class.

I'm not sure if those methods should be generated in bytecode, but $companionObject could have valid uses in runtime.

Note that it's not obvious how to generate those methods. Eg: generation $companionObject method is simple only if the returning value is static. Otherwise the object and the class instance could have different lifetime and this means that non-static classes would require a field to support this.

[smarter]

Fixed by #436.