book: revision of ch07 & finish atomics

Update #2

code/7/7.7.is.lock.free.cpp

@@ -6,6 +6,8 @@ struct A {
     int y;
     long long z;
 };
+
+int main() {
     std::atomic<A> a;
     std::cout << std::boolalpha << a.is_lock_free() << std::endl;
     return 0;