sys: alternative to bool(timer)

code/espurna/system.h

@@ -235,10 +235,14 @@ struct SystemTimer {
     SystemTimer(SystemTimer&&) = default;
     SystemTimer& operator=(SystemTimer&&) = default;
 
-    explicit operator bool() const {
+    bool armed() const {
         return _armed != nullptr;
     }
 
+    explicit operator bool() const {
+        return armed();
+    }
+
     void once(Duration duration, Callback callback) {
         start(duration, std::move(callback), false);
     }