Vendor dependencies

Let's see how I like this workflow.

vendor/tokio/tests/process_arg0.rs

@@ -0,0 +1,13 @@
+#![warn(rust_2018_idioms)]
+#![cfg(all(feature = "full", unix))]
+
+use tokio::process::Command;
+
+#[tokio::test]
+async fn arg0() {
+    let mut cmd = Command::new("sh");
+    cmd.arg0("test_string").arg("-c").arg("echo $0");
+
+    let output = cmd.output().await.unwrap();
+    assert_eq!(output.stdout, b"test_string\n");
+}