message: use std::string to implement make_shorty
  Summary: Replace std::ostringstream with std::string to save cpu cycles.
diff: @@ -1054,15 +1054,14 @@ void DexTypeList::gather_types(std::vector<DexType*>& ltype) const {
  }
  static DexString* make_shorty(const DexType* rtype, const DexTypeList* args) {
  - std::ostringstream ss;
  - ss << type_shorty(rtype);
  + std::string s;
  + s.push_back(type_shorty(rtype));
  if (args != nullptr) {
  for (auto arg : args->get_type_list()) {
  - ss << type_shorty(arg);
  + s.push_back(type_shorty(arg));
  }
  }
  - auto type_string = ss.str();
  - return DexString::make_string(type_string);
  + return DexString::make_string(s);
  }
  DexProto* DexProto::make_proto(const DexType* rtype, const DexTypeList* args) {
  