Change callback reform to not consume self (#779)

src/callback.rs

@@ -46,13 +46,14 @@ impl<IN> Callback<IN> {
 impl<IN: 'static> Callback<IN> {
     /// Changes input type of the callback to another.
     /// Works like common `map` method but in an opposite direction.
-    pub fn reform<F, T>(self, func: F) -> Callback<T>
+    pub fn reform<F, T>(&self, func: F) -> Callback<T>
     where
         F: Fn(T) -> IN + 'static,
     {
+        let this = self.clone();
         let func = move |input| {
             let output = func(input);
-            self.clone().emit(output);
+            this.emit(output);
         };
         Callback::from(func)
     }