message: Add some operators on Value
diff: @@ -272,6 +272,26 @@ impl AsRef<u64> for Value {
  }
  }
  +impl std::ops::Add for Value {
  + type Output = Value;
  + fn add(self, other: Value) -> Value {
  + Value(self.0.checked_add(other.0).unwrap())
  + }
  +}
  +
  +impl std::ops::Sub for Value {
  + type Output = Value;
  + fn sub(self, other: Value) -> Value {
  + Value(self.0.checked_sub(other.0).unwrap())
  + }
  +}
  +
  +impl std::ops::AddAssign for Value {
  + fn add_assign(&mut self, other: Value) {
  + self.0 = self.0.checked_add(other.0).unwrap();
  + }
  +}
  +
  #[cfg(test)]
  mod test {
  use super::*;
  