Vendor dependencies

Let's see how I like this workflow.

vendor/tokio/tests/support/signal.rs

@@ -0,0 +1,7 @@
+pub fn send_signal(signal: libc::c_int) {
+    use libc::{getpid, kill};
+
+    unsafe {
+        assert_eq!(kill(getpid(), signal), 0);
+    }
+}