parametric-functor

A known-parametric Functor typeclass.

As Oleg Grenrus argued in 2019, Functors should be parametric, and in particular should:

• Satisfy fmap coerce = coerce

• Have a ∀ a b. Coercible a b ⇒ Coercible (f a) (f b) super-constraint

Until this change gets adopted in base, this package can be used for a variant of Functor which satisfies these requirements.