add gsl::index typedef (#620)

* rename index in multi_span to span_index

gsl::index is name reserved for different type

* add gsl::index typedef

cppcoreguidelines referece: ES.107: Don't use unsigned for subscripts, prefer gsl::index

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Res-subscripts

* rename span_index to multi_span_index

tests/utils_tests.cpp

@@ -18,13 +18,20 @@
 
 #include <gsl/gsl_util> // for narrow, finally, narrow_cast, narrowing_e...
 
-#include <algorithm>  // for move
-#include <functional> // for reference_wrapper, _Bind_helper<>::type
-#include <limits>     // for numeric_limits
-#include <stdint.h>   // for uint32_t, int32_t
+#include <algorithm>   // for move
+#include <functional>  // for reference_wrapper, _Bind_helper<>::type
+#include <limits>      // for numeric_limits
+#include <stdint.h>    // for uint32_t, int32_t
+#include <type_traits> // for is_same
 
 using namespace gsl;
 
+TEST_CASE("sanity check for gsl::index typedef")
+{
+    static_assert(std::is_same<gsl::index, std::ptrdiff_t>::value,
+                  "gsl::index represents wrong arithmetic type");
+}
+
 void f(int& i) { i += 1; }
 
 TEST_CASE("finally_lambda")