https://github.com/microsoft/GSL/issues/1075
- Add `static_assert` because we only support C style array `at` for up to half of the address space.
- Add `std::` before `size_t`.
- tests:
- Add `#include <exception>`
- Implement `span_tests` `interop_with_gsl_at` like `at_tests` `std_span`
* move span specialization of 'at' to <gsl/span> and update the parameter to be taken by reference
* undid previous changes and acted upon decisions made in maintainer sync. Fixed tests failing in /kernel mode
* ran clang-format on the include folder
* ran clang-format on the test folder
Co-authored-by: Jordan Maples <jordan.maples@microsoft.com>
