gsl::at behavior change regarding gsl::span (#985)

* 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>

include/gsl/gsl_algorithm

@@ -1,3 +1,4 @@
 #pragma once
-#pragma message("This header will soon be removed. Use <gsl/algorithm> instead of <gsl/gsl_algorithm>")
+#pragma message(                                                                                   \
+    "This header will soon be removed. Use <gsl/algorithm> instead of <gsl/gsl_algorithm>")
 #include <gsl/algorithm>