progress on example 93 to demonstrate trapping all potential exceptions at compile time

removed constexpr from exception policies.
  this eliminates obstacle to gcc compilation which doesn't support constexpr throw
  unfortunately, all versions of gcc trip compiler fault so gcc not supported for now
safe_literal - make this an unsafe type since it doesn't have policies - this might change in the future

examples/example91.cpp

@@ -78,12 +78,13 @@ int main(){
     }
     catch(std::exception & e){
         std::cout << e.what() << '\n';
-        return 1;
+        // we expect to trap an exception
+        return 0;
     }
     catch(...){
         std::cout << "test interrupted\n";
         return 1;
     }
     std::cout << "end test\n";
-    return 0;
+    return 1;
 }