Vendor dependencies

Let's see how I like this workflow.

vendor/anyhow/tests/test_backtrace.rs

@@ -0,0 +1,13 @@
+#[rustversion::not(nightly)]
+#[ignore]
+#[test]
+fn test_backtrace() {}
+
+#[rustversion::nightly]
+#[test]
+fn test_backtrace() {
+    use anyhow::anyhow;
+
+    let error = anyhow!("oh no!");
+    let _ = error.backtrace();
+}