not_null constructor is now explicit (#659)

solves #395

include/gsl/pointers

@@ -72,7 +72,7 @@ public:
     static_assert(std::is_assignable<T&, std::nullptr_t>::value, "T cannot be assigned nullptr.");
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr not_null(U&& u) : ptr_(std::forward<U>(u))
+    constexpr explicit not_null(U&& u) : ptr_(std::forward<U>(u))
     {
         Expects(ptr_ != nullptr);
     }