Simplify pending work tracking (#1662)

Alan and I were talking in person about pending work tracking in the
Downstairs, and I found some room for simplification:

- Instead of tracking pending work and number of unmet dependencies in
  separate hashmaps, put them into a single `struct PendingJob`
- Instead of using `HashMap<JobId, IOop>` then cloning + sorting the
  keys to make a `Vec<JobId>`, just use a `VecDeque`

This are relatively small cleanups, but may have a small impact on
Downstairs RAM usage.

There are two behavior changes, both of which should be innocuous:

- We now check whether jobs are ready in the order that they're
  received, instead of sorting all pending jobs by `JobId` _then_ checking
  whether they're ready. This is fine, because readiness is determined by
  dependencies.
- The `show_work` debug function no longer sorts job IDs when printing
  pending work

Author: mkeeter