message: add missing serialization
diff: @@ -259,6 +259,12 @@ struct residue : public main_branch {
  residue(const main_branch& m)
  : main_branch(m) {
  }
  +
  + friend class boost::serialization::access;
  + template<class Archive>
  + void serialize(Archive& ar, const unsigned version) {
  + ar & boost::serialization::base_object<main_branch>(*this);
  + }
  };
  enum distance_type {
  