Vendor dependencies

Let's see how I like this workflow.

vendor/tokio-stream/tests/stream_once.rs

@@ -0,0 +1,12 @@
+use tokio_stream::{self as stream, Stream, StreamExt};
+
+#[tokio::test]
+async fn basic_usage() {
+    let mut one = stream::once(1);
+
+    assert_eq!(one.size_hint(), (1, Some(1)));
+    assert_eq!(Some(1), one.next().await);
+
+    assert_eq!(one.size_hint(), (0, Some(0)));
+    assert_eq!(None, one.next().await);
+}