06a956ad20
`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.