Storage service

[kovasb]

1. Large results should not be stored in datomic directly.
2. Some kinds of results are not values and can't be stored anyway
3. Conveying results to other services is easier by using a storage service, rather than asking the consumers to deal with datomic.

Therefore

1. Persisting values should be a secondary step after generating them. This can be a UI gesture.
   1b. What gets stored in datomic should only be a summary or representation of the result, sufficient for a human to see, understand, and take action on.
2. Must distinguish between persistent values, and results that are only available in the memory of the evaluator.
3. Need a concept of a storage service, and operations to transparently deal with values that may live in storage