Add `MCTransitionStackItem` to make per-transition data easier to conceptualize

[maxwellpirtle]

Discussion

With the addition of clock vectors, some state that should be associated with transitions in the transition stack (such as the clock vector associated with that transition) exists in the MCStateStackItem representing the state the transition moves the system into. For example, the clock vector associated with transition i lives in state i + 1. While this works, it makes it conceptually more difficult to understand where certain data associated with transitions "lives".

Proposal

The transition stack should consist of MCTransitionStackItems instead of MCTransitions directly. The items can be simply C-like structs similar to the MCStateStackItem which hold per-transition data such as clock vectors, as well as the transitions themselves. This will make finding clock vectors, as well as any other per-transition data we may want to add, conceptually simpler. I don't expect that this would be a difficult addition