Draft: Drop TypeNode reference counting

We don't allow unloading types, both static and dynamic, since 2013. The code that deals with reference counting is mostly dead code, and makes reasoning about the type system more complicated than necessary.

Since type classes and interfaces can only be instantiated, we introduce explicit getter functions that create a GTypeClass or a GTypeInterface vtable; the ref() and unref() API gets a "soft" deprecation, to allow people to progressively port their code.