Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/unique_ptr_as_mut.rs

@@ -0,0 +1,23 @@
+use cxx::UniquePtr;
+
+#[cxx::bridge]
+mod ffi {
+    struct Shared {
+        x: i32,
+    }
+
+    extern "C++" {
+        type Opaque;
+    }
+
+    impl UniquePtr<Shared> {}
+    impl UniquePtr<Opaque> {}
+}
+
+fn main() {
+    let mut shared = UniquePtr::<ffi::Shared>::null();
+    let _: &mut ffi::Shared = &mut shared;
+
+    let mut opaque = UniquePtr::<ffi::Opaque>::null();
+    let _: &mut ffi::Opaque = &mut opaque;
+}