UNSAFE: make Completion to be Send + Sync

core/io/completions.rs

@@ -23,6 +23,9 @@ pub struct Completion {
     pub(super) inner: Option<Arc<CompletionInner>>,
 }
 
+unsafe impl Send for Completion {}
+unsafe impl Sync for Completion {}
+
 #[derive(Debug, Default)]
 struct ContextInner {
     waker: Option<Waker>,