Indexed Foldable and Traversable

There seems to be a [sensible notion of a traversable type-aligned sequence](http://stackoverflow.com/questions/35123930/how-should-i-traverse-type-aligned-sequences), for traversing what McBride calls a `Path` in the context of an Atkey-style indexed applicative functor. There's also a related notion of folding one up in the context of a `Category`. This notion of traversability generalizes your `mapMR` and makes traversability stronger than mappability and foldability, as we'd expect.