This was a regression from 4469144891
The callback should be called, not only be called when the refcount
reaches 0. For example, the C# callback needs to know when the
refcount reaches 1, in order to swap to a weak GC handle.
These typedefs don't save much typing compared to the full `Ref<Resource>`
and `Ref<RefCounted>`, yet they sometimes introduce confusion among
new contributors.
* The harcoded 8 slots are no more and impose limits in the new extension system.
* New system is limitless, although it will impose small performance hit with a mutex.
* Use a token to request the instance binding.
**Warning**: Mono will most likely break as a result of this, will need to be modified to use the new system.
