export toDescList from Data.Map

[Evan Laforge]

> So since there's Foldable.foldl, I guess this isn't so pressing.  I
> still argue that it's more straightforward to simply export toDescList
> (why implement it and then not export it?) rather than make people
> roll their own with Foldable.

Oh, except the key part, of course.  So I guess we need toDescList
after all.  'foldr' and 'foldl' are also defined in the module so I
took the liberty of renaming them 'WithKey' and exporting foldlWithKey
(foldrWithKey is already exported as plain foldWithKey).  There's also
a foldlStrict, but I don't want to get too tangled up with all these
details when all I really want is toDescList.