`PersistBackend::is_empty` is removed. Instead, `load_from_persistence`
returns an option of the changeset. `None` means persistence is empty.
This is a better API than a separate method. We can now differentiate
between a persisted single changeset and nothing persisted.
`Store::aggregate_changeset` is refactored to return a `Result` instead
of a tuple. A new error type (`AggregateChangesetsError`) is introduced
to include the partially-aggregated changeset in the error. This is a
more idiomatic API.
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