Update useful
Add a note to the merge/make docstring
Remove separate phantom-merge test file.

It's obsolete since we ended up not using that implementation.
Remove some vestigial comments.

These were added to the file just as a todo/ideas list. Now that
we've done them, the options we were thinking about aren't really
relevant anymore.
Add a forwarding layer, that modifies IDs of a base layer.

We expect to use this for merging, so that a read of a tail node is
redirected to the head.
Make it easy to get a merge layer that forwards reads to the head id