Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/unique_ptr_to_opaque.rs

@@ -0,0 +1,23 @@
+mod outside {
+    #[repr(C)]
+    pub struct C {
+        pub a: u8,
+    }
+    unsafe impl cxx::ExternType for C {
+        type Id = cxx::type_id!("C");
+        type Kind = cxx::kind::Opaque;
+    }
+}
+
+#[cxx::bridge]
+mod ffi {
+    extern "C++" {
+        type C = crate::outside::C;
+    }
+
+    impl UniquePtr<C> {}
+}
+
+fn main() {
+    cxx::UniquePtr::new(outside::C { a: 4 });
+}