addressing a few more comments and adding gsl-std span compatibility tests

tests/utils_tests.cpp

@@ -23,7 +23,7 @@
 #include <limits>      // for numeric_limits
 #include <stdint.h>    // for uint32_t, int32_t
 #include <type_traits> // for is_same
-#include <cstddef>     // for std::size_t
+#include <cstddef>     // for std::ptrdiff_t
 
 using namespace gsl;