Merge branch 'entersub_for_cv'

* entersub_for_cv:
  Plan for t/generate.t.
  Document *_for_cv.
  Turn *_for_cv into classmethods.
  Require EntersubForCV 0.02.
  Rename _setup and _teardown to {setup,teardown}_for_cv.
  Return a nullop if the liftee returns a null sv.
  Port to B::Hooks::OP::Check::EntersubForCV.
  Create branch 'entersub_for_cv'.