build(rust): vendor rnix parser crate

third_party/crates/countme-3.0.1/README.md

@@ -0,0 +1,21 @@
+A library to quickly get the live/total/max counts of allocated instances.
+
+```rust
+#[derive(Default)]
+struct Widget {
+  _c: countme::Count<Self>,
+  ...
+}
+
+let w1 = Widget::default();
+let w2 = Widget::default();
+let w3 = Widget::default();
+drop(w1);
+
+let counts = countme::get::<Widget>();
+assert_eq!(counts.live, 2);
+assert_eq!(counts.max_live, 3);
+assert_eq!(counts.total, 3);
+
+eprintln!("{}", countme::get_all());
+```