Immutable values in dataflow graph

[bmitchell-lts]

At the moment, there are a handful of places where a value computed by the interpretation of p-code is modified before being committed to the graph. Unfortunately, this is not a perspicuous effect to the user which can lead to unexpected results that require a heavy amount of logging to diagnose.

This feature tracks a proposal to make values immutable after interpretation and commit to an "append-only" graph. If values are disputed by records or other sources of ground truth, additional operations that "correct" the underlying value should be emitted in order to retain the ability to audit the graph results without resorting to verbose logging.