split embassy-util into embassy-futures, embassy-sync.

embassy-futures/src/yield_now.rs

@@ -0,0 +1,25 @@
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Yield from the current task once, allowing other tasks to run.
+pub fn yield_now() -> impl Future<Output = ()> {
+    YieldNowFuture { yielded: false }
+}
+
+struct YieldNowFuture {
+    yielded: bool,
+}
+
+impl Future for YieldNowFuture {
+    type Output = ();
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        if self.yielded {
+            Poll::Ready(())
+        } else {
+            self.yielded = true;
+            cx.waker().wake_by_ref();
+            Poll::Pending
+        }
+    }
+}