Fixup images in ordered fashion

[vdice]

There currently is no ordering when ranging over the Images (map[string]bundle.Image) section of a bundle. See https://github.com/cnabio/cnab-to-oci/blob/master/remotes/fixup.go#L54-L60

Would we be amenable to introducing ordering (via sorting keys first, etc.) such that images are fixed up (pushed, resolved, etc.) in a reproducible/expected order?