htools: Switch to QuickCheck 2.x

Since current distros don't package anymore QuickCheck 1.x, let's move
to 2.x.

This requires also a few changes to the code:

- Test.QuickCheck.Batch doesn't exist anymore, so we need to write some
  scaffolding code to replace it
- the way test sizes are generated has changed, and we need to restrict
  (in some tests) the cluster size, as our code is not yet ready for
  hundreds of thousands of nodes in a cluster and we run out of stack
  (which could be a bug somewhere by itself, needs investigation)
- at least with GHC 7, floating point errors make a perfect cluster
  score even bigger, so we need to bump up the max. rounding error
  allowed

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>