* WIP: Provide --show-interface option to dump interfaces

Like ghcs own --show-iface this flag dumps a binary interface file to
stdout in a human (and machine) readable fashion. Currently it uses
json as output format.

* Fill all the jsonNull stubs

* Rework Bifunctor instance of DocH, update changelog and documentation

* replace changelog, bring DocMarkupH doc back

* Update CHANGES.md

* Update CHANGES.md

* Move Control.Arrow up

It would result in unused import if the Bifunctor instance is not generated.