A0-4006: Unit reconstruction refactor

[timorleph]

Reconstructing the explicit structure of the Dag from the, somewhat compressed, is now handled by a component named "reconstruction" rather than the cryptic "terminal". Internally it also consists of two separate pieces of logic – reconstructing the explicit hashes of parents and ensuring the order of units being added to the Dag.

This leaves a couple awkward points, which will hopefully be ironed out by further refactors around runway.

A big part of the motivation for this refactor is making the upcoming change to differently defined parents simpler to implement.

[github-actions[bot]]

Please make sure the following happened

• [ ] Appropriate tests created
• [ ] Infrastructure updated accordingly
• [ ] Updated existing documentation
• [ ] New documentation created
• [ ] Version bumped if breaking changes

[timorleph]

Added a small change, since DAG reconstruction does not actually need unit copies, and it likely won't be the component finally holding them; we will see in the next PRs though.