Proposal: Add concatMapM function (#2042)

[Don Stewart]

ndmitchell:
> Hi
> 
> >    1. In favor:
> >       - Twan van Laarhoven
> >       - Isaac Dupree
> >       - Don Stewart
> >       - Neil Mitchell
> >
> >    3. Opposed to any additions to the base library:
> >       - Bulat Ziganshin
> 
> I don't think 4 people is quite enough to overrule one persons
> objections, so I'd suggest that anyone reading this thread who does
> want this to go in (as it stands, without generalisation) should shout
> up now. I'd guess another couple of me-too's is enough to have a
> consensus.
> 

The problem is that the one objector is just fundamentally opposed to
the very process of adding stuff to the base library, for this case
and all others.

Given that the consensus is that we *will* continue to improve the 
base library, this objection isn't relevant -- it doesn't address
the proposal under consideration. 

Which leaves us with 4 in favour, and a few generalists.

-- Don