Method c_str

tests/catch.hpp:11203–11207

    }

    auto StringRef::c_str() const -> char const* {
        if( isSubstring() )
           const_cast<StringRef*>( this )->takeOwnership();
        return m_start;
    }
    auto StringRef::currentData() const noexcept -> char const* {
        return m_start;
    }