Document Durable Storage

[0xpatrickdev]

As a developer, I need to know how to make my contract and its related assets (objects, invitations, etc) durable across updates and restarts. I can see things like prepare, prepareExoClassKit, prepareExoClass, provideDurableMapStore used in some of the more recent contract examples, but I'm not entirely sure how they work or when to use them.

[dckc]

fixed in #968

https://docs.agoric.com/guides/zoe/contract-upgrade.html#durability

For stores, it has 3 successively more concise idioms, culminating in zone.mapStore(...).

For classes/kits/exos, it mostly just says

Use zone.exoClass() to define state and methods of kinds of durable objects ...

This presumes future work on reference api-docs will make it easy to find the rest of the zone API.