Vendor dependencies

Let's see how I like this workflow.

vendor/quote/tests/ui/does-not-have-iter-interpolated.rs

@@ -0,0 +1,9 @@
+use quote::quote;
+
+fn main() {
+    let nonrep = "";
+
+    // Without some protection against repetitions with no iterator somewhere
+    // inside, this would loop infinitely.
+    quote!(#(#nonrep)*);
+}