concrete functor and monad transformers
#34Migrate Bifoldable and Bitraversable instances for Constant from bifunctors
The Data.Bifoldable and Data.Bitraversable modules (originally from the bifunctors library) will be added to base in base-4.10 (GHC 8.2). They defined instances for Constant from transformers, so we should migrate those instances to transformers as well. I've implemented this change in http://hub.darcs.net/RyanGlScott/transformers/patch/50a7a371428cc90aa0ce886d255956a32c0e092b on my transformers fork.
