Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/ptr_unsupported.rs

@@ -0,0 +1,12 @@
+#[cxx::bridge]
+mod ffi {
+    unsafe extern "C++" {
+        type C;
+
+        fn get_ptr_to_reference() -> *mut &C;
+        fn get_uniqueptr_to_ptr() -> UniquePtr<*mut C>;
+        fn get_vector_of_ptr() -> UniquePtr<CxxVector<*mut C>>;
+    }
+}
+
+fn main() {}