feat: update mini tokio

__concurrent/async_study/examples/mini_tokio.rs

@@ -35,10 +35,7 @@ impl MiniTokio {
     }
 
     /// Spawn a future onto the mini-tokio instance.
-    fn spawn<F>(&mut self, future: F)
+    fn spawn<F>(&mut self, future: F) where F: Future<Output=()> + Send + 'static, {
-        where
-            F: Future<Output=()> + Send + 'static,
-    {
         self.tasks.push_back(Box::pin(future));
     }
 
@@ -62,9 +59,7 @@ struct Delay {
 impl Future for Delay {
     type Output = &'static str;
 
-    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>)
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<&'static str> {
-            -> Poll<&'static str>
-    {
         if Instant::now() >= self.when {
             println!("Hello world");
             Poll::Ready("done")