code motion: AbortOnDropJoinHandle lives in a mod just for now

at some point this will get factored to be tokio-specific but not just
now

crates/wasi/src/runtime/task.rs

@@ -0,0 +1,53 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+/// Exactly like a [`tokio::task::JoinHandle`], except that it aborts the task when
+/// the handle is dropped.
+///
+/// This behavior makes it easier to tie a worker task to the lifetime of a Resource
+/// by keeping this handle owned by the Resource.
+#[derive(Debug)]
+pub struct AbortOnDropJoinHandle<T>(tokio::task::JoinHandle<T>);
+impl<T> AbortOnDropJoinHandle<T> {
+    /// Abort the task and wait for it to finish. Optionally returns the result
+    /// of the task if it ran to completion prior to being aborted.
+    pub(crate) async fn cancel(mut self) -> Option<T> {
+        self.0.abort();
+
+        match (&mut self.0).await {
+            Ok(value) => Some(value),
+            Err(err) if err.is_cancelled() => None,
+            Err(err) => std::panic::resume_unwind(err.into_panic()),
+        }
+    }
+}
+impl<T> Drop for AbortOnDropJoinHandle<T> {
+    fn drop(&mut self) {
+        self.0.abort()
+    }
+}
+impl<T> std::ops::Deref for AbortOnDropJoinHandle<T> {
+    type Target = tokio::task::JoinHandle<T>;
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+impl<T> std::ops::DerefMut for AbortOnDropJoinHandle<T> {
+    fn deref_mut(&mut self) -> &mut tokio::task::JoinHandle<T> {
+        &mut self.0
+    }
+}
+impl<T> From<tokio::task::JoinHandle<T>> for AbortOnDropJoinHandle<T> {
+    fn from(jh: tokio::task::JoinHandle<T>) -> Self {
+        AbortOnDropJoinHandle(jh)
+    }
+}
+impl<T> Future for AbortOnDropJoinHandle<T> {
+    type Output = T;
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match Pin::new(&mut self.as_mut().0).poll(cx) {
+            Poll::Pending => Poll::Pending,
+            Poll::Ready(r) => Poll::Ready(r.expect("child task panicked")),
+        }
+    }
+}