message: Add must_use to add_iterator
diff: @@ -88,6 +88,7 @@ pub(crate) fn move_into_context<S: Storage, Q: Querier>(target: &mut Ctx, storag
  /// Add the iterator to the context's data. A new ID is assigned and returned.
  /// IDs are guaranteed to be in the range [0, 2**31-1], i.e. fit in the non-negative part if type i32.
  #[cfg(feature = "iterator")]
  +#[must_use = "without the returned iterator ID, the iterator cannot be accessed"]
  pub fn add_iterator<S: Storage, Q: Querier>(
  ctx: &mut Ctx,
  iter: Box<dyn Iterator<Item = StdResult<KV>>>,
  @@ -221,7 +222,7 @@ mod test {
  MockQuerier::new(&[(&HumanAddr::from(INIT_ADDR), &coins(INIT_AMOUNT, INIT_DENOM))]);
  move_into_context(instance.context_mut(), storage, querier);
  #[cfg(feature = "iterator")]
  - add_iterator::<S, Q>(instance.context_mut(), Box::new(std::iter::empty()));
  + let _ = add_iterator::<S, Q>(instance.context_mut(), Box::new(std::iter::empty()));
  }
  #[test]
  