Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/compiletest.rs

@@ -0,0 +1,9 @@
+#[allow(unused_attributes)]
+#[rustversion::attr(not(nightly), ignore)]
+#[cfg_attr(skip_ui_tests, ignore)]
+#[cfg_attr(miri, ignore)]
+#[test]
+fn ui() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/*.rs");
+}