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>

tests/utils_tests.cpp

@@ -16,14 +16,14 @@
 
 #include <gtest/gtest.h>
 
-#include <gsl/util>     // finally, narrow_cast
-#include <gsl/narrow>   // for narrow, narrowing_error
 #include <algorithm>   // for move
+#include <cstddef>     // for std::ptrdiff_t
 #include <functional>  // for reference_wrapper, _Bind_helper<>::type
+#include <gsl/narrow>  // for narrow, narrowing_error
+#include <gsl/util>    // finally, narrow_cast
 #include <limits>      // for numeric_limits
 #include <stdint.h>    // for uint32_t, int32_t
 #include <type_traits> // for is_same
-#include <cstddef>     // for std::ptrdiff_t
 
 using namespace gsl;
 
@@ -32,8 +32,7 @@ namespace
 void f(int& i) { i += 1; }
 static int j = 0;
 void g() { j += 1; }
-}
-
+} // namespace
 
 TEST(utils_tests, sanity_check_for_gsl_index_typedef)
 {
@@ -123,6 +122,7 @@ TEST(utils_tests, narrow_cast)
     EXPECT_TRUE(uc == 44);
 }
 
+#ifndef GSL_KERNEL_MODE
 TEST(utils_tests, narrow)
 {
     int n = 120;
@@ -145,3 +145,4 @@ TEST(utils_tests, narrow)
     n = -42;
     EXPECT_THROW(narrow<unsigned>(n), narrowing_error);
 }
+#endif // GSL_KERNEL_MODE