Vendor dependencies

Let's see how I like this workflow.

vendor/tokio/tests/signal_twice.rs

@@ -0,0 +1,22 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+#![cfg(unix)]
+
+mod support {
+    pub mod signal;
+}
+use support::signal::send_signal;
+
+use tokio::signal::unix::{signal, SignalKind};
+
+#[tokio::test]
+async fn twice() {
+    let kind = SignalKind::user_defined1();
+    let mut sig = signal(kind).expect("failed to get signal");
+
+    for _ in 0..2 {
+        send_signal(libc::SIGUSR1);
+
+        assert!(sig.recv().await.is_some());
+    }
+}