add map back to Functor

[Mario Blažević]

On 2019-02-08 1:45 p.m., Edward Kmett wrote:
>
> That sort of "magic" might be useful for making migration plans to get 
> sequenceA to be sequence and mapM and traverse without requiring folks 
> to memorize which one is in the class and which is a top level definition.

A similar sort of magic could be used for MonadOfNoReturn: we want to 
end up with a `return = pure` definition, not a method of any class, 
while all the existing code still compiles. Or is that migration plan 
already fully covered by existing magic tricks?