This moves away the loaders from the 'integrity' fields which is now dealt with lister
side. Instead the lister is send a dictionary of hex hashes.
This also improves the loader to check those checksums when retrieving the
artifact (content or tarball). Thanks to a bump in the swh.model version, this is now
able to deal with sha512 checksums checks as well.
This echoes with the ongoing work for the package loaders .