Rename Chunks to IterableOf, add IndexableOf

[mrocklin]

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.