Method swap

tests/fl/stl/type_traits.cpp:24–28

    int value;
    SwappableClass(int v = 0) : value(v) {}
    void swap(SwappableClass& other) {
        int tmp = value;
        value = other.value;
        other.value = tmp;
    }
};

FL_TEST_CASE("fl::integral_constant") {