Add an Iso for Nullable/Maybe

[paf31]

From #8. Something like

import Data.Profunctor

isoToMaybe :: forall p a b. Profunctor p => p (Nullable a) (Nullable b) -> p (Maybe a) (Maybe b)
isoToMaybe = dimap toNullable toMaybe 

[hdgarrood]

Presumably this would not be law-abiding because of the parametricity issue? Not that we shouldn't add this; it probably just wants documenting once we do get around to it.