Along with tests. Ref #56. In passing, also:
* Emit a message from the parent _update_ids() notifying the user what we're
* Add an `ON UPDATE CASCADE` expression to an FK that was missing it.
* Correct some pastos in the Plan docs.
* Set the plan and remove some diagnostics in `t/engine.t`.
Because, when we had the old IDs, things could be run in an order different
from the plan, the last run change may actually appear before other deployed
changes in the plan. So, as we updated the IDs, figure out which of the
deployed changes appears last in the plan, and use *that* change as the
I believe that this will cover the case of old IDs already, as
`$plan->get($id)` already works for old IDs, and we are already falling back
on trying to find a change by name and, where possible, tag. Ref #56.