Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/drop_shared.rs

@@ -0,0 +1,14 @@
+#[cxx::bridge]
+mod ffi {
+    struct Shared {
+        fd: i32,
+    }
+}
+
+impl Drop for ffi::Shared {
+    fn drop(&mut self) {
+        println!("close({})", self.fd);
+    }
+}
+
+fn main() {}