Class has_insert_key_value

tests/test_container_helpers.h:85–90

// Detect if type has insert(key, value) method (for maps)
template<typename T>
class has_insert_key_value {
    template<typename C> static auto test(int) -> decltype(fl::declval<C>().insert(0, fl::declval<fl::shared_ptr<int>>()), fl::true_type());
    template<typename> static fl::false_type test(...);
public:
    static constexpr bool value = fl::is_same<decltype(test<T>(0)), fl::true_type>::value;
};

// Detect if type has operator[] for assignment
template<typename T>