Add Enum type class

[kubukoz]

No description provided.

[Michaelt293]

See #1536

[johnynek]

I have taken a stab at this before with mixed success:

https://github.com/twitter/algebird/blob/develop/algebird-core/src/main/scala/com/twitter/algebird/Successible.scala

Some challenges:

1. what if you are countable, but not bounded? e.g. BigInteger
2. what if you are upper bounded but not lower bounded?

If you have something unbounded, you want def next(a: A): A, but if you are bounded, you want def next(a: A): Option[A].

As for a law, I think it is something like: a < next(a) and there is nothing in between: forAll { (a, b) => if (a < b) (b == next(a) || next(a) < b) else true

[sideeffffect]

this ticket can be closed
cc @LukaJCB @travisbrown

[LukaJCB]

good call, thanks @sideeffffect!