HasResolver for enum is not monadic

[teh]

In all our other HasResolver instances the Handler type is m x, for for enum it's not.

instance forall m ksN enum. (Applicative m, API.GraphQLEnum enum) => HasResolver m (API.Enum ksN enum) where
  type Handler m (API.Enum ksN enum) = enum

should probably be

  type Handler m (API.Enum ksN enum) = m enum

Backwards incompatible change though, so we'd need an 0.2 release