message: hide an impl behind the iterator feature
diff: @@ -51,6 +51,7 @@ pub trait StorageIterator {
  }
  }
  +#[cfg(feature = "iterator")]
  impl<I: StorageIterator + ?Sized> StorageIterator for Box<I> {
  fn next(&mut self) -> FfiResult<NextItem> {
  (**self).next()
  