message: Use checked_sub to implement Sub for Uint128
diff: @@ -263,11 +263,9 @@ impl ops::Sub for Uint128 {
  fn sub(self, other: Self) -> StdResult<Self> {
  let (min, sub) = (self.u128(), other.u128());
  - if sub > min {
  - Err(StdError::underflow(min, sub))
  - } else {
  - Ok(Uint128(min - sub))
  - }
  + min.checked_sub(sub)
  + .map(Uint128)
  + .ok_or_else(|| StdError::underflow(min, sub))
  }
  }
  