Write out ruler metadata state when serializing ruler objects

As of #121, when serializing rulers, the ids of each ruler are written out to json. When loading the json and constructing the rulers again, the ids are used to re-derive the state of the metadata generators, so that the user can pick up where they left off with annotating. This approach could be improved upon by serializing the metadata generators themselves, instead of re-deriving their state using just the ids.

Start by tracing the ruler-object JSON serialization and loading paths, then inspect how metadata generators derive their state from ruler IDs. Define how generator state should be serialized and restored, and verify that annotation can resume with the same metadata-generator state after a save and reload.

Written by the indexing model from the issue text.