message: add get_output_total for Tx
diff: @@ -284,6 +284,13 @@ impl Tx {
  pub fn add_output(&mut self, o: TxOut) {
  self.outputs.push(o)
  }
  + pub fn get_output_total(&self) -> Coin {
  + let mut total = Coin::zero();
  + for ref o in self.outputs.iter() {
  + total = (total + o.value).unwrap()
  + }
  + total
  + }
  }
  impl raw_cbor::se::Serialize for Tx {
  fn serialize(&self, serializer: Serializer) -> raw_cbor::Result<Serializer> {
  