Should the marray subscript operator be constexpr?

The subscript operators on `std::array` is declared [`constexpr` since C++17](https://en.cppreference.com/w/cpp/container/array/operator_at). Should the subscript operator of `sycl::marray` follow suit or is there a reason to no make it `constexpr`?

Depending on the conclusion, this could be extended to `sycl::vec` too.

Start by reviewing the sycl::marray subscript operator and comparing it with std::array's constexpr declaration since C++17. Determine whether marray should follow that behavior, and whether the same conclusion applies to sycl::vec; done means documenting or specifying a clear decision.

Written by the indexing model from the issue text.