Test Parallel / Map replay

Context

Map and Parallel [will] use an efficient replay mechanism to avoid executing everything again once a parent context has completed.

Task

Write tests for map and parallel to ensure that replays are consistent and properly rebuild the result without executing again.

Acceptance Criteria

1. Replay mechanism is checked with deterministic branches
2. Replay mechanism is checked with non-deterministic branching
3. We ensure that the correct result is rebuilt
4. We ensure that we don't execute the parent object
5. We test with payloads that are sufficiently big that they require bottom up reconstruction of result instead of loading up from checkpoint.