Vendor dependencies

Let's see how I like this workflow.

vendor/cfg-if/tests/xcrate.rs

@@ -0,0 +1,14 @@
+cfg_if::cfg_if! {
+    if #[cfg(foo)] {
+        fn works() -> bool { false }
+    } else if #[cfg(test)] {
+        fn works() -> bool { true }
+    } else {
+        fn works() -> bool { false }
+    }
+}
+
+#[test]
+fn smoke() {
+    assert!(works());
+}