Updatable/mergeable indices

Now that we're using index files as described in #1, there are some features we can additonally implement. One of them is to update an archive.

As I wrote in #1,

> Should we ever want to add data to the archive, we can append to the content file, and write a new index.

It might even be useful to merge index files, as there is already a [script to do](https://github.com/BurntSushi/fst/blob/255c66ac66baafd8cf24d3ad792e553fe73e22c1/fst-bin/src/merge.rs) that in fst.

Start by reading the index-file approach described in issue #1, then inspect fst-bin/src/merge.rs for the referenced index-merging script. Define how an archive should be updated and how index files should be merged; done means both operations are supported without breaking the existing archive format.

Written by the indexing model from the issue text.