message: elle.serialization.Serializer: Upsert hooks with only one lookup.
diff: @@ -1434,15 +1434,8 @@ namespace elle
  TypeMap&
  _map()
  {
  - auto tn = type_info<T>();
  - auto& hm = hierarchy_map();
  - auto it = hm.find(tn);
  - if (it == hm.end())
  - {
  - hm[tn] = TypeMap();
  - return boost::any_cast<TypeMap&>(hm[tn]);
  - }
  - return boost::any_cast<TypeMap&>(it->second);
  + auto insertion = hierarchy_map().emplace(type_info<T>(), TypeMap());
  + return boost::any_cast<TypeMap&>(insertion.first->second);
  }
  static std::unordered_map<TypeInfo, std::string>&
  