make explicit not_null move constructor so it is noexcept (#630)

* make explicit not_null move constructor so it is noexcept

* added test for not_null being noexcet move constructible

include/gsl/pointers

@@ -82,6 +82,7 @@ public:
     {
     }
 
+    not_null(not_null&& other) = default;
     not_null(const not_null& other) = default;
     not_null& operator=(const not_null& other) = default;