Deprecated Data.Aeson.Generic

The module is deprecated for the following reasons:

- It has issues: #75, #84, #101, #107.

- It's slow compared to GHC Generics and very, very, very slow
  compared to TH. See some recent benchmark results in #116.

- The encoding/decoding can't currently be configured using the
  encoding Options.

- Related to the previous point: GHC Generics and TH use compatible
  encoders/decoders (if the same Options have been
  used). Data.Aeson.Generic does things slightly differently.

- The original author is not around to work on it.