Export insertion index

[willscott]

the insertion index is currently in an internal subpackage https://github.com/ipld/go-car/blob/master/v2/internal/store/insertionindex.go#L31

if consumers want to be able to combine multiple other indices, they will also want an insertion index. this is a re-usable utility, and we don't get much from hiding it from external use.