model: set reads_from "return value" in model-checker

model: set reads_from "return value" in model-checker

Previously, values (let alone the reads-from relationship) were not actually
returned from the model-checker to the user. This step sets up the return value
so that the user context can retrieve it rather than using a value stuck in the
snapshotting memory.

There are still several TODOs along with the reads-from relationship, but this
code is stable enough for providing a basis for further work.