build(rust): vendor rnix parser crate

third_party/crates/countme-3.0.1/examples/print_at_exit.rs

@@ -0,0 +1,11 @@
+#[derive(Default)]
+struct Widget {
+    _t: countme::Count<Self>,
+}
+
+fn main() {
+    let w1 = Widget::default();
+    let _w2 = Widget::default();
+    drop(w1);
+    let _w3 = Widget::default();
+}