message: Expain why pointer equality works for BuiltinFn
  Summary: I investigated three times why and always kept forgetting.
diff: @@ -26,6 +26,9 @@ pub(crate) struct BuiltinFn(pub(crate) FrozenValue);
  impl PartialEq<FrozenValue> for BuiltinFn {
  fn eq(&self, other: &FrozenValue) -> bool {
  + // Pointer equality works because `#[starlark_module]` proc macro
  + // generates a singleton which allocates the function only once
  + // even if builder function is called multiple times.
  self.0.to_value().ptr_eq(other.to_value())
  }
  }
  