Suggestion: Polykinded On + OnTarget class

[gusbicalho]

This splits functionality on On into a typeclass to allow adding new types that implement targets for it. The typeclass dispatches on a kind, so we can still have the general "Symbol implies HasField" behaviour. I then took advantage of that to add:

• and instance for pairs of targets
• an Nth type that can be used to index fields by their position, using GHC Generics.

I can see how this may end up overcomplicating the design, but the end result looks nice to me. Any other field accessors will now only need to define a single instance.

I have very little experience with GHC Generics, so I don't know if there's a better way to write the code in Deriving.On.Nth.