Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/lifetime_extern_rust.rs

@@ -0,0 +1,17 @@
+#[cxx::bridge]
+mod ffi {
+    extern "Rust" {
+        type Opaque;
+        fn f<'a>(&'a self, arg: &str) -> &'a str;
+    }
+}
+
+pub struct Opaque;
+
+impl Opaque {
+    fn f(&self, _arg: &str) -> &str {
+        ""
+    }
+}
+
+fn main() {}