message: add IntoIterator for the TxPayload
diff: @@ -72,6 +72,13 @@ impl TxPayload {
  TxPayload::new(Vec::new())
  }
  }
  +impl IntoIterator for TxPayload {
  + type Item = <Vec<tx::TxAux> as IntoIterator>::Item;
  + type IntoIter = <Vec<tx::TxAux> as IntoIterator>::IntoIter;
  + fn into_iter(self) -> Self::IntoIter {
  + self.txaux.into_iter()
  + }
  +}
  impl ::std::ops::Deref for TxPayload {
  type Target = [tx::TxAux];
  fn deref(&self) -> &Self::Target { self.txaux.deref() }
  