Add a function similar to Gen.resultOf to Cogen

[nigredo-tori]

Currently to build a Cogen for some type (A) that is isomorphic to another type (B) for which a Cogen exists, we need to summon the Cogen[B], and contramap it. This leads to noisy code like this:

implicit val InstantCogen: Cogen[Instant] =
  Cogen[(Long, Int)].contramap(i => (i.getEpochSecond, i.getNano))

It should be easy to add Cogen.resultOf that would derive a Cogen[A] given a function A => B (and the implicit Cogen[B] in scope), similarly to what Gen.resultOf does. We would then be able to write:

implicit val InstantCogen: Cogen[Instant] =
  Cogen.resultOf(i => (i.getEpochSecond, i.getNano))

[non]

We merged #590 to master. Thanks! Closing.