Explore state migration options

[prevwong]

Consumers of Reka will likely store the State data structure in a database of some kind. However, we should anticipate that any existing State structure may become invalid in the following events :-

• The schema of the State type changes in a future release
• External globals/components that consumers of Reka expose to their end-users may become deprecated or renamed; thus any existing references to those externals in the State will become undefined.

Hence, we should consider approaches in handling migrations to prevent existing State to become invalid or unusable.