Vendor dependencies

Let's see how I like this workflow.

vendor/proc-macro2/tests/features.rs

@@ -0,0 +1,8 @@
+#[test]
+#[ignore]
+fn make_sure_no_proc_macro() {
+    assert!(
+        !cfg!(feature = "proc-macro"),
+        "still compiled with proc_macro?"
+    );
+}