export toDescList from Data.Map

[Christian Maeder]

Chris Kuklewicz wrote:
> I conjecture that the logic, if any exists, behind having the two names
> is so that the Data.Map API can be applied to several implementations. 
> And in another implementation it may be most efficient or most lazy to
> return elements in unsorted order.  So there is a "toList" which asks
> for the easiest conversion to a list, and a "toAscList" which asks for
> the sorted version.

if "toList" returned "elements in unsorted order" it would break the
abstraction: equal maps could return different list. So toList must
either be toAscList or toDescList.

> 
> As for this proposal, I vote to have toDescList and foldlWithKey.  This
> would likely simplify one of my uses of Data.Map.

+1 and foldrWithKey.

Christian