Method optional

Bcore/src/main/cpp/Dobby/tests/catch.hpp:5853–5856

  }

  auto optional() -> DerivedT & {
    m_optionality = Optionality::Optional;
    return static_cast<DerivedT &>(*this);
  };

  auto required() -> DerivedT & {
    m_optionality = Optionality::Required;