Most of the way towards Traversals

This mish-mash of stuff should get us a lot of the power that will be needed once `Traversal`s become possible to express directly.  It's also useful on its own!

- `ArrayZipper`:  A zipper for `Array`, duh!
- `IxMultiStore`: A "manually inlined" version of the `IxStoreT` comonad transformer as applied to `ArrayZipper`.
- Some additional extensions to `Array`, including the ever-useful `foldRight`!