Fix GDecomp#redecorate

[adelbertc]

• Redecoration includes the focus of the decomposition when acting on
  the rest of the graph
• Fix GDecomp generator so incorrect instances cannot be built (this
  should probably be a smart constructor)

/cc @runarorama

[adelbertc]

Some notes:

• The new redecorate fails comonad laws without my fix to the generator
• Both the old and new redecorate pass the comonad laws fine (at least it hasn't failed on a couple of runs on my machine)
• The old one kinda makes more sense since you're already focused on a particular node, it's a bit weird to re-attach the focused node before running the action on the rest of the graph
• At the same time the new one is useful and "consistent", running an action on each node given the decomposition of the graph with respect to it (see: http://blog.higher-order.com/blog/2016/04/02/a-comonad-of-graph-decompositions/)

[joroKr21]

GraphGen.genGDecomp is not fixed properly. It can still generate a graph that doesn't contain the vertex (or the adjacent nodes) in the context. Here is a proper solution:

  def genGDecomp[N: Arbitrary, A: Arbitrary, B: Arbitrary]: Gen[GDecomp[N, A, B]] = for {
    g <- graphGen[N, A, B] if !g.isEmpty
    n <- Gen.oneOf(g.nodes)
  } yield g.decomp(n).toGDecomp.get

[rossabaker]

Thanks! Folded into #27 so we can minimize release churn.