spec: adding step change

[josef-widder]

This PR adds the logic that checks checks whether a change in the consensus step variable (e.g., from prevote to precommit) enabled a consensus input that needs to be fired.

• Add a function StepChange called by the driver to see whether consensus needs to be called after the step has changed (e.g., from prevote to precommit) and calls it if necessary
• Add a wrapper around the old driver function (now called driverLogic) that records a step change
• Add branches that detect a step change, and call

For later reference, here is the Google doc by Anca where she listed the cases.

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[hvanz]

Looks good to me.