`deploy.Plan.Apply` was only consumed by the engine, and seemed to be in
the wrong place given the API exported by the rest of `Plan` (i.e.
`Plan.Start` + `PlanIterator`). Furthermore, we were missing a reasonable
opportunity to share code between `update` and `preview`, both of which
need to walk the plan. These changes move the plan walk into `package engine`
as `planResult.Walk` and replace the `Progress` interface with a new interface,
`StepActions`, which subsumes the functionality of the former and adds support
for implementation-specific step execution. `planResult.Walk` is then
consumed by both `Engine.Deploy` and `Engine.PrintPlan`.
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