`gsl::not_null` should allow returning by value for the types whose size equals to the size of two pointers

[ozaktash]

In gsl::not_null, objects of types smaller than two pointers can be returned by value, but when the size equals to the size of two pointers, returning by reference is forced:

    template<typename T>
    using value_or_reference_return_t = std::conditional_t<
                                            sizeof(T) < 2*sizeof(void*) && std::is_trivially_copy_constructible<T>::value,
                                            const T,
                                            const T&>;

I think returning by value should also be allowed for types whose size equals to the size of two pointers:

    template<typename T>
    using value_or_reference_return_t = std::conditional_t<
                                            sizeof(T) <= 2*sizeof(void*) && std::is_trivially_copy_constructible<T>::value,
                                            const T,
                                            const T&>;

This is the threshold used in the current cppfront and is also compliant with C++ Core Guidelines.

[carsonRadtke]

Thanks for reporting this issue! I agree, it looks like this should be <=. From the guidelines:

(Simple) ((Foundation)) Warn when a parameter passed by reference to const has a size less or equal than 2 * sizeof(void*). Suggest passing by value instead.

ozaktash, do you want to submit a PR?