1075 Wrong Expects in gsl::at? (#1076)

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`

tests/at_tests.cpp

@@ -20,6 +20,7 @@
 
 #include <array>            // for array
 #include <cstddef>          // for size_t
+#include <exception>        // for terminate
 #include <initializer_list> // for initializer_list
 #include <vector>           // for vector
 #if defined(__cplusplus) && __cplusplus >= 202002L