message: Implement plus_seconds via plus_nanos
diff: @@ -24,8 +24,7 @@ impl Timestamp {
  }
  pub const fn plus_seconds(&self, addition: u64) -> Timestamp {
  - let nanos = Uint64::new(self.0.u64() + addition * 1_000_000_000);
  - Timestamp(nanos)
  + self.plus_nanos(addition * 1_000_000_000)
  }
  pub const fn plus_nanos(&self, addition: u64) -> Timestamp {
  