Unbundled functors, as a typeclass decorating the object-level function.
A unbundled functor.
F : C → D (just a function) has
we can write
map F f : F X ⟶ F Y for the action of
F on a morphism
f : X ⟶ Y.
Bundle a functorial function as a functor.
G ∘ F is a functorial if both