A smart pointer type that can wrap either a &'static T or an Arc<T>.
ArcRef<T> implements PartialEq by checking pointer identity first. If two
ArcRefs point at the same backing allocation or the same &'static reference,
they compare equal without calling T's PartialEq implementation. Otherwise,
equality falls back to comparing the dereferenced values.
This means cloned ArcRefs are always equal to each other, and equality remains
reflexive even for wrapped types whose PartialEq is not, such as f64::NAN.
See the examples/ folder for example usage.