dyn_array_iterator lacks the random access iterator operators it declares #1270

dyn_array_iterator declares iterator_category = std::random_access_iterator_tag but supplies none of operator<, operator>, operator<=, operator>=, operator-> or n + it, and its default constructor exists only when __cpp_lib_ranges is defined. Any algorithm that relies on the declared category fails to compile:

gsl::dyn_array<int> a{3, 1, 2};
std::sort(a.begin(), a.end());        // error: no match for operator<
bool b = a.begin() < a.end();          // same
gsl::dyn_array<int>::iterator it{};    // error before C++20

Reproduces on main (417ef68) with GCC 15 at C++14, 17 and 20 (the default constructor case at 14 and 17). A default constructor is part of the forward iterator requirements in every standard the library supports, and the relational operators, -> and n + it are part of the random access requirements.