add a layer to the architecture with a DB api between the FS and ent?

This issue grew out of the discussion in issue
08f0d7ee7842c439382816d21ec1dea2.

Currently the entomologist crate code directly reads and writes files
and calls `git commit` when it wants.  The directory is managed by the
application, generally as an ephemeral worktree containing a (possibly
detached) checkout of the `entomologist-data` branch.

We may be able to simplify the ent internals (and the application)
by adding a "database" API between ent and the filesystem.

There is some preliminary design brainstorming in the issue mentioned
above.