Callback cleaner: context can be taken from timer

src/timer.hpp

@@ -14,21 +14,22 @@ namespace sc {
         uint64_t _id {};
         double _time {};
         bool _repeat {};
-        void (*_expired_func)(timer* self, void* context);
+        void (*_expired_func)(timer* self);
         void* _context {};
 
         static void callback(union sigval);
 
         public:
-            timer(void(*expired_func)(timer*, void*),
-                  double time,
+            timer(double time,
                   bool repeat,
-                  void* context);
+                  void(*expired_func)(timer*),
+                  void* context = nullptr);
             ~timer();
 
             uint64_t id() const { return _id; }
             double time() const { return _time; }
             bool repeat() const { return _repeat; }
+            void* context() const { return _context; }
 
             bool is_armed() const;
             void time_left(struct itimerspec& cur_value) const;