Drop Generate class in favour of Scalacheck's Arbitrary

[Howy Perrin]

I'm not convinced that our Generate typeclass is really general enough - there probably isn't a way to encapsulate all types of generation into one typeclass (for instance, Scalameter needs a different API)

So the plan is to delete Generate and instead have a sub-module which depends on Scalacheck and provides arbitraries for all Ropes. This will also entail moving all of the tests into a separate module.