Add QuickCheck test for Kleene idempotence axiom and implement function
for generating arbitrary TCP packets.
Implement QuickCheck tests for remaining Kleene algebra axioms.
Fix bug in Semantics.eval, make QuickCheck less verbose

* The case for start in Semantics.eval was wrong: we were not
  correctly unioning the accumulator

* The NetKAT_Tests QuickCheck tests were using the QuickCheck.verbose
  configuration, which was printing a lot of extra output to the
  console. Changed these to QuickCheck.quick instead.

* Some minor tidying...
Factor common code out of Kleene axiom tests.
Remove unnecessary resize calls, turn off verbose mode.