message: Add back Borrow for FrozenRef/Box
  Summary: Accidentally removed in (https://github.com/facebookexperimental/starlark-rust/commit/840d5e285a55b46bee68e3e3bc1e5a331f7240e5)
diff: @@ -79,6 +79,12 @@ impl<T: ?Sized> Borrow<T> for FrozenRef<T> {
  }
  }
  +impl<T: ?Sized> Borrow<T> for FrozenRef<Box<T>> {
  + fn borrow(&self) -> &T {
  + &*self
  + }
  +}
  +
  impl<T: ?Sized> PartialEq for FrozenRef<T>
  where
  T: PartialEq,
  