Implement PartialOrd for HashSet

cc #15294

Does it make sense to just implement PartialOrd for every type that implements Set? There is a well-defined partial order on sets based on set inclusion. I'm thinking something like this:

    impl<T, S: Set<T>> PartialOrd for S {
        fn partial_cmp(&self, other: &S) -> Option<Ordering> {
            match (self.is_subset(other), other.is_subset(self)) {
                (true, true) => Some(Equal),
                (true, false) => Some(Less),
                (false, true) => Some(Greater),
                (false, false) => None
            }
        }
    }