Vendor dependencies

Let's see how I like this workflow.

vendor/tokio/tests/io_async_read.rs

@@ -0,0 +1,10 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+
+use tokio::io::AsyncRead;
+
+#[test]
+fn assert_obj_safe() {
+    fn _assert<T>() {}
+    _assert::<Box<dyn AsyncRead>>();
+}