Description of issue or feature request:

As per the specification (v1.0.1) length and hashes fields in timestamp and snapshot metadata are optional. The reference implementation should reflect this.

Current behavior:

• timestamp metadata always includes length and hashes of snapshot metadata (see reposoitory_lib.generate_timestamp_metadata)
• snapshot metadata always excludes length and hashes of (delegated) targets metadata (see repository_lib.generate_snapshot_metadata)

Expected behavior:
Add option to include/exclude these fields in the relevant functions in repository_lib. Since these functions are usually called via write/writeall the option should be made available there too. It's also conceivable to configure this via attributes on the Repository object.