`IteratorBase::SaveInternal`, and `DatasetBase::CheckExternalState`
        become pure-virtual and subclasses are now expected to provide an
        implementation.
    *   The deprecated `DatasetBase::IsStateful` method is removed in favor of
        `DatasetBase::CheckExternalState`.
    *   Deprecated overrides of `DatasetBase::MakeIterator` and
        `MakeIteratorFromInputElement` are removed.