This commit gets all our law tests working with a new, more robust
ScalaCheck version. It currently relies upon an unreleased Discipline
snapshot, so it definitely won't build in Travis.

There were a bunch of test bugs that got fixed, mostly around
foldRight/reduceRight consistency for non-empty data structures.

One law violation was uncovered: MonadCombine[Option] doesn't pass the
left-distributivity law we wrote. We should figure out whether this
represents a real law violation and remove the instance, or whether
the law is too strict somehow.

One counter-example is:

    def f(x: Int): Option[Int] = if (x == 0) None else Some(x)
    val a = Option(0)
    val b = Option(1)
    (a <+> b).flatMap(f) != (a.flatMap(f) <+> b.flatMap(f))

Review by @adelbertc et al. We will need to wait for a new Discipline
release before merging this, and we'll need to do something about the
MonadCombine laws/instances (currently the MonadCombine[Option] test
is commented out).

Also remove various obsolete/commented code.

We provide some Cogen instances which were not included with 1.13.2.
These instances will be provided in a future release, so when they
become available we should remove ours.

This commit also removes one instance that wasn't in 1.13.1 but was
available in 1.13.2: Cogen[Stream[A]].