Add instance Monad ZipList

[Henning Thielemann]

On Thu, 4 Jun 2020, Carter Schonwald wrote:

> *i believe* the way to write the monad instance would be to implement a join :: SizedLIst n (SizedList n a) ->
> SizedList n a 
> that picks the diagonal. But i could be wrong? it wasn't a priority for me at the time, but would that "diagonal"
> / trace be the right way to induce a bind? 

Sure, that's analogous to instance Monad ((->) a).