Generic instances for FromJSON/ToJSON

[ulysses4ever]

The very convenient Generics machinery from aeson is not present here. Is there a good reason for this? Would a PR adding it be welcomed?

[hvr]

Definitely welcomed and appreciated! I just never found the time to do it myself. I'm happy to provide assistance if needed :-)

[andreasabel]

I guess this issue blocks more wide-spread adoption of microaeson.
Maybe one can copy some code from https://github.com/haskell/aeson/blob/b296cb96b7e1338dc3eb4ad1a157f77d150a001f/src/Data/Aeson/Types/FromJSON.hs.

[ulysses4ever]

The current aeson implementation looks a bit too smart, and it makes use of libraries (e.g. tagged, which allows ConsFromJSON; there's probably more, that's just the first thing came up in my experiment with it). I'm also unsure about the arity polymorphism in GToJSON/GFromJSON. I'm thinking maybe taking aeson pre-1.0 would be a better match for microaeson.