Change the signature of `Container.serialize` and
`Container.deserialize` to use `Buffer` instead of `bytes`, as this
makes the functions compatible with the `revng::pypeline::Buffer`
classes.
When calling `StorageProvider.invalidate`, return a dictionary that
conveys to the caller which objects were deleted from storage as a
consequence of the paths supplied to the `invalidate` call.
Fix the invalidation logic in both `InMemoryStorageProvider` and
`Sqlite3StorageProvider`, as it did not account for propagating the
invalidation to both children objects and parent objects.
Giacomo Vercesi