Class has_insert_position

tests/test_container_helpers.h:419–427

// Detect if type has insert(const_iterator, value)
template<typename T>
class has_insert_position {
    template<typename C> static auto test(int) -> decltype(
        fl::declval<C>().insert(fl::declval<C>().begin(), 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 erase(const_iterator)
template<typename T>