Added <*> syntax to transform Gen[Seq[X]] into gen[Seq[Y]].

Moved identityG from a function to an enhancement and added some type-variance magic.
Added liftL and liftR for (A, B) to promote a left or right generator value.
Added a sequence operation to lift Seq[Gen[T]] to Gen[Seq[T]].
Added double.probability e.g. 0.1.probability that returns a Gen[Boolean] with those odds of True.