code: func (s *FileStorage) StoreUser(email string, data *UserData) error {
  	err := os.MkdirAll(s.user(email), 0700)
  	if err != nil {
  		return fmt.Errorf("making user directory: %v", err)
  	}
  	err = ioutil.WriteFile(s.userRegFile(email), data.Reg, 0600)
  	if err != nil {
  		return fmt.Errorf("writing user registration file: %v", err)
  	}
  	err = ioutil.WriteFile(s.userKeyFile(email), data.Key, 0600)
  	if err != nil {
  		return fmt.Errorf("writing user key file: %v", err)
  	}
  	return nil
  }
docs: // StoreUser implements Storage.StoreUser by writing it to disk. The base
  // directories needed for the file are automatically created as needed.