Arrow: Hash typeclass

[pakoito]

We have equals in Eq, toString in Show. Let's do hashCode in Hash!

https://github.com/typelevel/cats/blob/master/kernel/src/main/scala/cats/kernel/Hash.scala

[pt2121]

i'd like to take a stab at this

[1Jajen1]

As per #795 I think I'll attempt this. Is the old branch still worth using, or should I start fresh. On that note: Are there some guides to contributing? Or is it best to just start and go along with it in a pr? :)

[pakoito]

The contributing guide is...incomplete at best. You need:

• The typeclass
• A set of Laws/Tests
• An instance for at least a couple of datatypes, specially wrappers like Id, ListK, SetK, MapK, SequenceK...
• Apply the laws to the instances in the IdTest, ListTest, etc.

And lastly, the docs. Ping me when you're ready for this:

• An entry on the docs menu
• An entry in the typeclass intro
• Its own doc

[pakoito]

Closed by #1138