Cannot turn off PathPiece and ToHttpApiData newtype instances for keys

[lf-]

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).

persistent/persistent/Database/Persist/TH.hs

Lines 1748 to 1750 in 7222127

	deriving newtype instance ToHttpApiData (BackendKey $(pure backendT)) => ToHttpApiData (Key $(pure recordType))
	deriving newtype instance FromHttpApiData (BackendKey $(pure backendT)) => FromHttpApiData(Key $(pure recordType))
	deriving newtype instance PathPiece (BackendKey $(pure backendT)) => PathPiece (Key $(pure recordType))