Cannot turn off PathPiece and ToHttpApiData newtype instances for keys #1458

Hi!

I'm writing a not-a-webapp and I think it's somewhat odd that Persistent will generate deriving newtype instance FromHttpApiData and likewise for PathPiece for all keys, even though this should not be necessary for non-web users; furthermore there is a mechanism in MkPersistSettings to set mpsDeriveInstances to derive more instances, which could be used to achieve this for web users that want it.

I think that the default mkPersistSettings should probably be changed to include these extra instances, such that it can be overridden if they are not desirable. This would unfortunately be a breaking change for those overriding mpsDeriveInstances while discarding the default value (which was previously empty-list).