Add more WriterT instances

[ceedubs]

WriterT currently has Monad and Eq instances, but there are many more that it could have. Some examples that I think can be added are:

• Functor (Add WriterT instances for supertypes of Monad #628)
• Apply (Add WriterT instances for supertypes of Monad #628)
• Applicative (Add WriterT instances for supertypes of Monad #628)
• FlatMap (Add WriterT instances for supertypes of Monad #628)
• MonadFilter (Add more WriterT instances #696)
• Alternative (Add more WriterT instances #696)
• MonadCombine (Add more WriterT instances #696)
• ApplicativeError (Add WriterT MonadError instance #689)
• MonadError (Add WriterT MonadError instance #689)
• Show (#607: Add missing Show instances #928)
• Foldable
• Traverse
• Bifunctor (Add WriterT Bifunctor #673)
• CoflatMap (Adding CoflatMap and tests to WriterT and fixed laws for CoflatMap #1049)
• Comonad
• Semigroup (Adding WriterT Monoid #1014)
• Monoid (Adding WriterT Monoid #1014)
• SemigroupK (Add more WriterT instances #696)
• MonoidK (Add more WriterT instances #696)

[ceedubs]

I'll work on this.

[akilegaspi]

Is this still in progress? I'm thinking of contributing for this enhancement.

[kailuowang]

that'll be awesome. I updated the checklist, three instances still missing.

[akilegaspi]

Just a quick question.

Is the resolving type for the WriterT[F, L, A] after passing through extract is F[(L, A)]

[barambani]

I would look into this one if that's ok.

[akilegaspi]

I think this should be closed since @barambani already finished it and it has been merged to master