Allow more flexibility in job priorities for scheduling

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Our current representation of priority for a job is, effectively, a time instant.

This works well enough for many scheduling algorithms, but in some cases is insufficient. Notably, it prevents the release! function from specifying anything about how priority ties are broken.

On the overly-flexible end, we could change the priority type to Any. This would allow types such as Tuple to be set as job priorities, so that ties in the first element are broken by subsequent elements.

Or probably better, we could add another type parameter for a job's priority.

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Closed for archiving.