Registers the given instance (already created).
        :param class_type: The class type.
        :param instance: The instance to register.
        :param register_as: The types to register the class as, defaults to the given class_type.

def register_instance(self, class_type, instance, register_as=None):
        """
        Registers the given instance (already created).
        :param class_type: The class type.
        :param instance: The instance to register.
        :param register_as: The types to register the class as, defaults to the given class_type.
        """
        registration = _InstanceRegistration(instance)
        self._register(class_type, registration, register_as)