Rename Chunks to IterableOf, add IndexableOf

The `Chunks` type is really just anything over which we can iterate. It doesn't actually have to be used in a chunking workflow. It might be best to rename it appropriately.

Separately, we might want to make an `IndexableOf` type that supports both iteration and also indexing. Whereas `IterableOf(a)`takes an iterable or a function `f :: -> [a]`, `IndexableOf(a)` might take either an indexable (like a list) or a function `f :: int -> a` and a number of elements. `IndexableOf` might also be extended to something n-dimensional, so possibly `f :: [int] -> a` and a shape rather than a number of elements.