[WiP] StepExecution

[Trello](https://trello.com/c/QFTIBCof/82-need-stepfactory)

Blocking issues:
* (none known to remain)

Noncritical outstanding tasks as I see them:
* Collapse `Step`, `StepDescriptor`, and `StepExecution` implementations into one compilation unit for clarity.
* See if `ExecutorStep`’s `PlaceholderTask.runningTasks` can be replaced by `StepExecutionIterator`.
* Ditto `WatchYourStep`’s `activeWatches`.
* Ditto `SegmentStep`’s `segmentsByNameByJob`.