Reintroduce generic Message type for canvas::Program

As it is useful to make the `Message` completely free in many
implementations.

examples/bezier_tool/src/main.rs

@@ -104,9 +104,7 @@ mod bezier {
         curves: &'a [Curve],
     }
 
-    impl<'a> canvas::Program for Bezier<'a> {
-        type Message = Curve;
-
+    impl<'a> canvas::Program<Curve> for Bezier<'a> {
         fn update(
             &mut self,
             event: Event,

examples/clock/src/main.rs

@@ -76,9 +76,7 @@ impl Application for Clock {
     }
 }
 
-impl canvas::Program for Clock {
-    type Message = Message;
-
+impl<Message> canvas::Program<Message> for Clock {
     fn draw(&self, bounds: Rectangle, _cursor: Cursor) -> Vec<Geometry> {
         let clock = self.clock.draw(bounds.size(), |frame| {
             let center = frame.center();

examples/color_palette/src/main.rs

@@ -235,9 +235,7 @@ impl Theme {
     }
 }
 
-impl canvas::Program for Theme {
-    type Message = Message;
-
+impl<Message> canvas::Program<Message> for Theme {
     fn draw(&self, bounds: Rectangle, _cursor: Cursor) -> Vec<Geometry> {
         let theme = self.canvas_cache.draw(bounds.size(), |frame| {
             self.draw(frame);

examples/game_of_life/src/main.rs

@@ -328,9 +328,7 @@ mod grid {
         }
     }
 
-    impl<'a> canvas::Program for Grid {
-        type Message = Message;
-
+    impl<'a> canvas::Program<Message> for Grid {
         fn update(
             &mut self,
             event: Event,

examples/pure/game_of_life/src/main.rs

@@ -394,8 +394,7 @@ mod grid {
         }
     }
 
-    impl canvas::Program for Grid {
-        type Message = Message;
+    impl canvas::Program<Message> for Grid {
         type State = Interaction;
 
         fn update(

examples/solar_system/src/main.rs

@@ -128,9 +128,7 @@ impl State {
     }
 }
 
-impl canvas::Program for State {
-    type Message = Message;
-
+impl<Message> canvas::Program<Message> for State {
     fn draw(
         &self,
         bounds: Rectangle,