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;
 

tests/notnull_tests.cpp

@@ -327,3 +327,5 @@ TEST_CASE("TestNotNullCustomPtrComparison")
     CHECK((NotNull1(p1) >= NotNull2(p2)) == (p1 >= p2));
     CHECK((NotNull2(p2) >= NotNull1(p1)) == (p2 >= p1));
 }
+
+static_assert(std::is_nothrow_move_constructible<not_null<void *>>::value, "not_null must be no-throw move constructible");