Vendor dependencies

Let's see how I like this workflow.

vendor/tokio/tests/io_util_empty.rs

@@ -0,0 +1,32 @@
+#![cfg(feature = "full")]
+use tokio::io::{AsyncBufReadExt, AsyncReadExt};
+
+#[tokio::test]
+async fn empty_read_is_cooperative() {
+    tokio::select! {
+        biased;
+
+        _ = async {
+            loop {
+                let mut buf = [0u8; 4096];
+                let _ = tokio::io::empty().read(&mut buf).await;
+            }
+        } => {},
+        _ = tokio::task::yield_now() => {}
+    }
+}
+
+#[tokio::test]
+async fn empty_buf_reads_are_cooperative() {
+    tokio::select! {
+        biased;
+
+        _ = async {
+            loop {
+                let mut buf = String::new();
+                let _ = tokio::io::empty().read_line(&mut buf).await;
+            }
+        } => {},
+        _ = tokio::task::yield_now() => {}
+    }
+}