Re: export toDescList from Data.Map

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.