This gives us the ability to take a D-initialized object and pass it to Janet without moving any data around in particular. The void* should be a pointer given by janet_unwrap_abstract on a Janet which was earlier made by janet_wrap_abstract on an object of type T. It should be used in any static functions that will be expected for registerType on the void* that's sent in.
Allows one to register a JanetAbstractType with Janet.