Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/vector_autotraits.rs

@@ -0,0 +1,21 @@
+use cxx::CxxVector;
+
+#[cxx::bridge]
+mod ffi {
+    extern "C++" {
+        type ThreadSafe;
+        type NotThreadSafe;
+    }
+
+    impl CxxVector<ThreadSafe> {}
+    impl CxxVector<NotThreadSafe> {}
+}
+
+unsafe impl Send for ffi::ThreadSafe {}
+
+fn assert_send<T: Send>() {}
+
+fn main() {
+    assert_send::<CxxVector<ffi::ThreadSafe>>();
+    assert_send::<CxxVector<ffi::NotThreadSafe>>();
+}