initial macos porting work

lib/runtime/src/sys/macos/channel.rs

@@ -0,0 +1,10 @@
+//! macOS channel wake helpers.
+
+use crate::op::completion::{CompletionFuture, CompletionHandle, completion};
+use crate::platform::current::runtime::try_current_thread_handle;
+
+pub(crate) fn runtime_waiter<T: Send + 'static>() -> (CompletionFuture<T>, CompletionHandle<T>) {
+    let owner = try_current_thread_handle()
+        .expect("async channel operations must be polled on a runtime thread");
+    completion(owner)
+}