Add index mutation methods to pkg/v1/mutate

[jonjohnsonjr]

It's currently really difficult to programmatically create an image index.

This is mostly my fault because I had tunnel vision when implementing pkg/v1/layout, but we need these to work on an abritrary ImageIndex.

Ideally, we could do something like:

idx := empty.Index
idx, _ = mutate.AppendImage(idx, foo)
idx, _ = mutate.AppendImage(idx, bar)

Which would produce an index that references both images foo and bar. I'd like the same options to work, and we'll probably want AppendIndex as well.

I suspect we'd be able to refactor most of the layout package implementations to use the mutate package without making it inefficient.