Better Representation of Assignments (maybe Value Summaries)

Right now we use ITEs, which are straightforward for sequential merges but become very annoying when we do parallel merges (we need to tear down the ITE into what I think are value summaries, and then reconstruct an ITE after moving concretes up front).

Find a better encoding, perhaps kevins idea of `Action::Assign(guard, value, dont_care_guard)` as a value summary

Start by tracing how assignments are represented with ITEs and how sequential and parallel merges process them. Compare that flow with the proposed Action::Assign(guard, value, dont_care_guard) value-summary encoding; done means parallel merges no longer require tearing down and reconstructing ITEs.

Written by the indexing model from the issue text.