Generic ToJSONKey and FromJSONKey

[Lysxia]

There seems to be no easy way to write common instances of ToJSONKey and FromJSONKey. In the following answer I use Show and Read but that still takes some typing and knowledge about the library.

https://stackoverflow.com/questions/56609201/how-to-use-a-sum-type-as-a-key-in-map-when-using-aeson/56610450#56610450

We should be able to derive this with Generic, defining a gToJSONKeyText and gFromJSONKeyTextParser. Related to issue #463 which suggests this solution as an intermediate step for a harder problem.