Add a WIP thread pool example

examples/thread-pool.rs

@@ -0,0 +1,15 @@
+fn main() {
+    // Start a threadpool.
+    // for _ in 0..num_cpus::get().max(1) {
+    //     thread::spawn(|| smol::run(future::pending()));
+    // }
+
+    // Start a stoppable threadpool.
+    // let mut pool = vec![];
+    // for _ in 0..num_cpus::get().max(1) {
+    //     let (s, r) = oneshot::channel<()>();
+    //     pool.push(s);
+    //     thread::spawn(|| smol::run(async move { drop(r.await) }));
+    // }
+    // drop(pool); // stops the threadpool!
+}