Convert any hashable type into a much smaller Hashed<T> which still supports checking equality.

All that is stored inside the Hashed<T> is the u64 hash of the type, so this can save a lot of space over storing objects themselves, while being more convenient than hashing manually and with added type safety. The downside is, of course, the only thing you can do with the resulting Hashed<T> is check if it is equal to another Hashed<T>. No magic here.