message: Implement sum with reference rhs using add
diff: @@ -363,7 +363,7 @@ impl Sum for Uint128 {
  impl<'a> Sum<&'a Uint128> for Uint128 {
  fn sum<I: Iterator<Item = &'a Self>>(iter: I) -> Self {
  - iter.fold(Uint128::zero(), |a, b| a + *b)
  + iter.fold(Uint128::zero(), ops::Add::add)
  }
  }
  