Add folds

[treeowl]

There are analogues to foldMap, foldr, and foldl available. The foldMap is particularly pleasant; it's pretty much like the usual one, but maps into a Category instead of a Monoid. The foldr and foldl can be implemented in terms of the foldMap, but I haven't yet figured out how to do so efficiently; direct implementations are not hard though. Let me know if you want me to hack up the code (I've only done it in Idris, for things like Okasaki's implicit deques, but it should be straightforward to translate).

[treeowl]

I've just worked out a way (although probably not the best way) to express a notion of traversability for type-aligned-sequence-like things. In order to express both maps and folds, the Applicative of Traversable needs to be replaced by some sort of indexed Applicative. My first (awkward and seemingly backwards) approach is at http://stackoverflow.com/questions/35123930/how-should-i-traverse-type-aligned-sequences but I bet there's some way to make things line up a bit more sensibly.

[treeowl]

Actually, that approach turns out not to be backwards at all for your version of TAList, so I'll try to put together a pull request. I also realized we can make

newtype IxBackwards f i j a = IxBackwards { ixForwards :: f j i a }

to traverse backwards.