message: packer: add helper to get bytes from codec
diff: @@ -51,6 +51,12 @@ impl<R: std::io::BufRead> Codec<R> {
  self.0.read_exact(&mut buf)?;
  Ok(u128::from_be_bytes(buf))
  }
  + #[inline]
  + pub fn get_bytes(&mut self, n: usize) -> std::io::Result<Vec<u8>> {
  + let mut buf = vec![0u8; n];
  + self.0.read_exact(&mut buf)?;
  + Ok(buf)
  + }
  }
  impl<W: std::io::Write> Codec<W> {
  #[inline]
  