Vendor dependencies

Let's see how I like this workflow.

vendor/tokio/tests/join_handle_panic.rs

@@ -0,0 +1,20 @@
+#![warn(rust_2018_idioms)]
+#![cfg(all(feature = "full", not(tokio_wasi)))] // Wasi does not support panic recovery
+
+struct PanicsOnDrop;
+
+impl Drop for PanicsOnDrop {
+    fn drop(&mut self) {
+        panic!("I told you so");
+    }
+}
+
+#[tokio::test]
+async fn test_panics_do_not_propagate_when_dropping_join_handle() {
+    let join_handle = tokio::spawn(async move { PanicsOnDrop });
+
+    // only drop the JoinHandle when the task has completed
+    // (which is difficult to synchronize precisely)
+    tokio::time::sleep(std::time::Duration::from_millis(3)).await;
+    drop(join_handle);
+}