message: use a map for Attributes
diff: @@ -130,6 +130,9 @@ mod cbor {
  pub fn cbor_array_start(nb_elems: usize, buf: &mut Vec<u8>) {
  write_length_encoding(MajorType::ARRAY, nb_elems, buf);
  }
  + pub fn cbor_map_start(nb_elems: usize, buf: &mut Vec<u8>) {
  + write_length_encoding(MajorType::MAP, nb_elems, buf);
  + }
  }
  @@ -346,7 +349,11 @@ impl Attributes {
  }
  impl ToCBOR for Attributes {
  fn encode(&self, buf: &mut Vec<u8>) {
  + cbor::cbor_map_start(2, buf);
  + // TODO
  + cbor::cbor_uint_small(0, buf);
  self.derivation_path.encode(buf);
  + cbor::cbor_uint_small(1, buf);
  self.stake_distribution.encode(buf)
  }
  }
  