Function swap

include/behaviortree_cpp/contrib/expected.hpp:1397–1403

        std17::is_swappable<U>::value
    )
    swap( unexpected_type & other ) noexcept (
        std17::is_nothrow_swappable<U>::value
    )
    {
        using std::swap;
        swap( m_error, other.m_error );
    }

    // TODO: ??? unexpected_type: in-class friend operator==, !=