This drops support for type hierarchy instance creators. I don't expect this to be a problem. We'll also detect fewer errors where multiple type adapters can serialize the same type. With APIs like getNextTypeAdapter, I think this might actually be an improvement!
Fixed previously broken tests to verify the behavior that a hierarchical type adapter for a sub-class can be registered after registering a hierarchical type adapter for the base-class. The vice-versa is not allowed as it would result in hiding the sub-class hierarchical type adapter.