In the example_cli we were not always committing (seemingly by mistake).
This then caused all the examples to have to compensate by manually
committing.
`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.
Previously, regardless of whether writing to persistence backend is
successful or not, the logic always cleared `self.staged`. This is
changed to only clear `self.staged` after successful write.
Additionally, the written changeset (if any) is returned, and
`PersistBackend::write_changes` will not be called if `self.staged` is
empty.
This is a more generic version of `keychain::persist::*` structures.
Additional changes:
* The `Append` trait has a new method `is_empty`.
* Introduce `Store` structure for `bdk_file_store`.
