message: Replace map + flatten with flat_map.
diff: @@ -884,12 +884,11 @@ where
  .enumerate()
  .zip(&their_index_panorama)
  .map(create_message)
  - .map(|msgs| {
  + .flat_map(|msgs| {
  msgs.into_iter().map(|msg| {
  ProtocolOutcome::CreatedTargetedMessage(msg.serialize(), sender.clone())
  })
  })
  - .flatten()
  .collect()
  }
  }
  