Poor SparseCat iteration for containers without indices

[Zachary Sunberg]

It's too hard for the compiler to figure out iterate for SparseCat when the containers are not things that can be indexed. This can result in type instability. This is not a huge problem since most of the time the containers in SparseCat are arrays or tuples that can be accessed by index.

Using the zip iterator or some ideas from it may fix it.