Make types more flexible

[treeowl]

I realized in coercible-utils #14 that the inference-enhancing effects of the O type are only needed for some of the arguments. For the rest, things can be left much freer. That suggests a modification here as well:

class Isomorphic n o where
  packI   :: o -> n
  default packI :: (Generic n, GNewtype (Rep n), O n ~ GO (Rep n), o ~ O n) => o -> n
  packI = to . gpack

  unpackI :: n -> o
  default unpackI :: (Generic n, GNewtype (Rep n), O n ~ GO (Rep n), o ~ O n) => n -> o
  unpackI = gunpack . from

class Isomorphic n (O n) => Newtype n where
  type O n :: *
  type O n = GO (Rep n)

pack :: Newtype n => O n -> n
pack = packI

unpack :: Newtype n => n -> O n
unpack = unpackI

Now you can use Isomorphic, rather than Newtype, for the type variables that are already constrained by the type of the packer the user passes in.

[treeowl]

This is not backwards-compatible, of course, but it seems (almost) strictly better.