* Decide a way to test methods that are only visible given a specific
  command-line option. For example, Kernel#gsub with -n/-p on 1.9.
* Look at automating discovery of guarded bugs which have been fixed.
* Use mocks for all Math functions that coerce with #to_f; currently a fixture
  is used.
* investigate slow specs (run with -fp) and make them faster.
* restore some caller specs from 642bf529
* restore refinements specs and update. See 56c5528f and f20a62e8.