Make context mutable

the user callback gets a const timer ref, so in principle the context
would always be read-only. However, in practice it would often be
desirable to mutate the passed context. Therfore, it's now mutable.

src/timer.hpp

@@ -10,12 +10,12 @@ namespace sc {
         static uint64_t next_id;
         static std::mutex mutex;
 
-        timer_t _tid;
         uint64_t _id {};
         double _time {};
         bool _repeat {};
         void (*_expired_func)(const timer& self);
-        void* _context {};
+        mutable void* _context {};
+        timer_t _tid;
 
         static void callback(union sigval);