Proposal: Add concatMapM function (#2042)

[Neil Mitchell]

Hi

> > I think we should ignore the generalization to
> > genericSuperDuperConcatMapMXYZ for now :).
>
> Why?

Orthogonality. It is silly to have have superConcatMapM, but not
superMapM. If that wants doing it should be a separate proposal after
this one.

Thanks

Neil