message: Use ref mut less
  I couldn't fix mark_as_acked here because that falls squarely in
  borrow-checker territory (I'm surprised that it doesn't complain as-is,
  but whatever).
diff: @@ -497,11 +497,8 @@ impl SendStream {
  };
  if fin {
  - if let SendStreamState::DataSent {
  - ref mut fin_sent, ..
  - } = self.state
  - {
  - *fin_sent = true
  + if let SendStreamState::DataSent { fin_sent, .. } = &mut self.state {
  + *fin_sent = true;
  }
  }
  }
  @@ -522,8 +519,9 @@ impl SendStream {
  send_buf.mark_as_acked(offset, len);
  if fin && send_buf.buffered() == 0 {
  self.conn_events.send_stream_complete(self.stream_id);
  - self.state
  - .transition(SendStreamState::DataRecvd { final_size });
  + self.state.transition(SendStreamState::DataRecvd {
  + final_size,
  + });
  }
  }
  _ => qtrace!("mark_as_acked called from state {}", self.state.name()),
  @@ -536,11 +534,8 @@ impl SendStream {
  };
  if fin {
  - if let SendStreamState::DataSent {
  - ref mut fin_sent, ..
  - } = self.state
  - {
  - *fin_sent = false
  + if let SendStreamState::DataSent { fin_sent, .. } = &mut self.state {
  + *fin_sent = false;
  }
  }
  }
  @@ -619,9 +614,9 @@ impl SendStream {
  let buf = &buf[..can_send_bytes as usize];
  - let sent = match self.state {
  + let sent = match &mut self.state {
  SendStreamState::Ready => unreachable!(),
  - SendStreamState::Send { ref mut send_buf } => send_buf.send(buf),
  + SendStreamState::Send { send_buf } => send_buf.send(buf),
  _ => return Err(Error::FinalSizeError),
  };
  @@ -633,7 +628,7 @@ impl SendStream {
  }
  pub fn close(&mut self) {
  - match self.state {
  + match &mut self.state {
  SendStreamState::Ready => {
  self.state.transition(SendStreamState::DataSent {
  send_buf: TxBuffer::new(),
  @@ -641,7 +636,7 @@ impl SendStream {
  fin_sent: false,
  });
  }
  - SendStreamState::Send { ref mut send_buf } => {
  + SendStreamState::Send { send_buf } => {
  let final_size = send_buf.retired + send_buf.buffered() as u64;
  let owned_buf = mem::replace(send_buf, TxBuffer::new());
  self.state.transition(SendStreamState::DataSent {
  