Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/enum_overflows.rs

@@ -0,0 +1,17 @@
+#[cxx::bridge]
+mod ffi {
+    enum Good1 {
+        A = 0xFFFF_FFFF_FFFF_FFFF,
+    }
+    enum Good2 {
+        B = 0xFFFF_FFFF_FFFF_FFFF,
+        C = 2020,
+    }
+    enum Bad {
+        D = 0xFFFF_FFFF_FFFF_FFFE,
+        E,
+        F,
+    }
+}
+
+fn main() {}