deriveJSONGADT requires toJSON and parseJSON to be in scope

[ryantrinkle]

The TH should not be looking up names from the calling module's scope. Instead, they should use the names from the TH implementation's scope. The impact is that if the user does not have Data.Aeson in scope, the TH splice fails.

Here are two examples of where it's wrong:

• aeson-gadt-th/src/Data/Aeson/GADT/TH.hs

  Line 116 in b8bebee

  impl <- funD (mkName "toJSON")

• aeson-gadt-th/src/Data/Aeson/GADT/TH.hs

  Line 149 in b8bebee

  parser <- funD (mkName "parseJSON")

Those are the only two I noticed.