time: add a tick() function to Periodically to avoid needing Fn

src/time.rs

@@ -23,9 +23,17 @@ impl Periodically {
     }
 
     pub fn run<F>(&mut self, f: F) where F: FnOnce() {
-        if self.last_run.elapsed() >= self.duration {
+        if self.tick() {
             f();
+        }
+    }
+
+    pub fn tick(&mut self) -> bool {
+        if self.last_run.elapsed() >= self.duration {
             self.last_run = Instant::now();
+            true
+        } else {
+            false
         }
     }
 }