Hash typeclass improvements

[1Jajen1]

I made a few changes to the Hash typeclass to make it both easier to define and better to use.

• Removed the Eq superclass from it. It never made sense and it should not have been there in the first place.
• Added hashWithSalt. Allows for two nice things: First hashing with salt, but also allows very easy definitions for sum types: Either<L, R>.hash(HL: Hash<L>, HR: Hash<R>): Int = fold({ l -> HL.run { l.hashWithSalt(0) } }, { r -> HR.run { r.hashWithSalt(1) } })
• Added Hashed datatype. It carriers its precomputed hash and thus provides very fast inequality checks and provides its Hash instances with the cached hash.

Overall this makes Hash actually somewhat useful over the default hashCode now :)

[raulraja]

Great stuff for hashing 🙌 thanks @1Jajen1

[rachelcarmena]

I removed the last merge with master branch because errors raised after solving the conflict. In this way, the conflict can be solved again @1Jajen1 😉