Make it easier to derive an instance for `HasArguments`.

[soenkehahn]

This should be dead simple and very short.

[soenkehahn]

Currently it is very noisy to use record types with withCli (see for example here: https://github.com/soenkehahn/getopt-generics/blob/master/docs/SimpleRecord.hs), because you have to

• switch on DeriveGeneric,
• import WithCli,
• import GHC.Generics,
• derive GHC.Generics.Generic,
• derive Generics.SOP.Generic,
• derive Generics.SOP.HasDatatypeInfo,
• derive HasArguments.

For a library that strives for ease of use this is far too complicated. There are a few options of removing some of the noise:

• Use TemplateHaskell to derive all the needed instances.
• Switch to pure GHC.Generics, thereby removing the need to derive the sop classes.
• Switch to Data.Data.Data.
• Remove the need for HasArguments (and just implement everything directly on Generic).
• Use a type family to compute a constraint that contains all needed classes and put that in the deriving line.

Though I haven't looked into any of these in detail, so I don't know how any of them would look in practice or if they are even feasable. (Especially for the last one I would be surprised if ghc allowed to do that.)

(See also https://github.com/well-typed/generics-sop/issues/10.)

[soenkehahn]

Resolved by #50 using Generics.SOP.GGP. (@kosmikus, @sol)