Vendor dependencies

Let's see how I like this workflow.

vendor/tokio/tests/io_read_exact.rs

@@ -0,0 +1,15 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+
+use tokio::io::AsyncReadExt;
+use tokio_test::assert_ok;
+
+#[tokio::test]
+async fn read_exact() {
+    let mut buf = Box::new([0; 8]);
+    let mut rd: &[u8] = b"hello world";
+
+    let n = assert_ok!(rd.read_exact(&mut buf[..]).await);
+    assert_eq!(n, 8);
+    assert_eq!(buf[..], b"hello wo"[..]);
+}