Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/reference_to_reference.rs

@@ -0,0 +1,13 @@
+#[cxx::bridge]
+mod ffi {
+    unsafe extern "C++" {
+        type ThingC;
+        fn repro_c(t: &&ThingC);
+    }
+    extern "Rust" {
+        type ThingR;
+        fn repro_r(t: &&ThingR);
+    }
+}
+
+fn main() {}