async: split async_engine into multiple files

src/async_engine/ae_yield.rs

@@ -0,0 +1,27 @@
+use {
+    crate::async_engine::ae_queue::DispatchQueue,
+    std::{
+        future::Future,
+        pin::Pin,
+        rc::Rc,
+        task::{Context, Poll},
+    },
+};
+
+pub struct Yield {
+    pub(super) iteration: u64,
+    pub(super) queue: Rc<DispatchQueue>,
+}
+
+impl Future for Yield {
+    type Output = ();
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        if self.queue.iteration() > self.iteration {
+            Poll::Ready(())
+        } else {
+            self.queue.push_yield(cx.waker().clone());
+            Poll::Pending
+        }
+    }
+}