...With `public_key` as primary key.

We're keeping this until the first release after devashishdxt/idb#36 is
resolved.

Also, remove `indexmap` dependency, because we don't need it anymore.

We don't need it anymore, we have its features on our idb fork now.

Also move it into a place where the author of a new migration is more
likely to be reminded to update it.

Here it's still useful.

It's very helpful when debugging wasm tests.

…action

We cannot use the regular `Database::open()` because this would result
in infinite recursion when used in migrations.

This would result in infinite recursion

…sion