Method swap

tests/catch.hpp:11197–11201

    }

    void StringRef::swap( StringRef& other ) noexcept {
        std::swap( m_start, other.m_start );
        std::swap( m_size, other.m_size );
        std::swap( m_data, other.m_data );
    }

    auto StringRef::c_str() const -> char const* {
        if( isSubstring() )