Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/enum_unsatisfiable.rs

@@ -0,0 +1,9 @@
+#[cxx::bridge]
+mod ffi {
+    enum Bad {
+        A = -0xFFFF_FFFF_FFFF_FFFF,
+        B = 0xFFFF_FFFF_FFFF_FFFF,
+    }
+}
+
+fn main() {}