Added SOP.Generically and a SOP.Generic instance for GHC.Generically. Updated documentation.

[Icelandjack]

For #153, this adds:

• generically (package) as a dependency, to support versions before GHC.Generically was added (base 4.17).
• An SOP.Generic (GHC.Generically a) instance.

  instance (GHC.Generic a, GFrom a, GTo a, All SListI (GCode a)) => Generic (GHC.Generically a) where
    type Code (GHC.Generically a) = GCode a
  
    from :: GHC.Generically a -> Rep (GHC.Generically a)
    from (GHC.Generically a) = gfrom a
  
    to :: Rep (GHC.Generically a) -> GHC.Generically a
    to rep = GHC.Generically (gto rep)

• An SOP.HasDatatypeInfo (GHC.Generically a) instance.
• A SOP Generically newtype for attaching generic SOP definitions:

  newtype Generically a = Generically a

All in all deriving SOP.Generic can be done GHC.Generically and deriving SOP instances can be done SOP.Generically:

data Ok = Ok Int Int Int
  deriving
  stock GHC.Generic

  deriving (SOP.Generic, HasDatatypeInfo)
  via GHC.Generically
  
  deriving (Eq, NFData, ..)      -- assuming we have instances of SOP.Generically
  via SOP.Generically

The next step should be writing SOP generic instances for SOP.Generically, for type classes in base.

[Icelandjack]

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