Support for capability-style records about barbies

[jcpetruzza]

I think you are definitely right, and we should be able to handle this case. Conceptually, the main thing that we are missing is to allow the functor argument (e.g. m in LogCap m) to occur under another functor when deriving FunctorB (and similarly for the other classes). At the moment we allow a barbie type to occur under a functor (e.g. Maybe (LogCap m)), but not the functor argument itself. This shouldn't be hard to fix.

Because we have an instance Functor (-> r), this addition alone would let us handle simpler capability records such as:

data LogCap (m :: Type -> Type)
  { logInfo  :: String -> m ()
  , logError :: String -> m ()
  }
  deriving (Generic)

instance FunctorB LogCap

Of course, one would like to handle functions with an arbitrary number of parameters (not just one) or, more generally, an unlimited number of nested functors. This is in fact a bit tricky; but ultimately doable, I think (e.g., one could identify arbitrary chains of functor applications in the generic representation of the type, coerce the type into a version where all the functors were smashed into one using Compose, operate on that single functor, and then coerce it back to the original representation).

A quicker workaround, would be to do a manual unrolling defining k instances of the form "the functor argument occurs under {1,2,3,...k} functors".