cleanup natvis (#1077)

- `fail_fast`,`basic_string_span` and `basic_zstring_span` are not declared in the headers any more.
- Name the exporting header file for all types in `GSL.natvis`.

tests/assertion_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include "deathTestCommon.h"
-#include <gsl/assert> // for fail_fast (ptr only), Ensures, Expects
+#include <gsl/assert> // for Ensures, Expects
 #include <gtest/gtest.h>
 
 using namespace gsl;