Support forced ranking for "high" workers

[mmccool]

Problems:

• If you just have "high" and "low", devs will abuse it (mark everything as high)
• In practice, there are a limited number of high performance threads available
• We can't necessarily share the number of high performance threads with the dev (fingerprinting risk, maps to number of P-cores, reveals details of platform, etc).

Idea: Use forced ranking for "high" workers:

• Within Workers marked as "high", devs need to place them in order from most to least important
• This can be done with a number, e.g. 1, 2, 3 - with no duplicates, lowest number most important
• Workers would be assigned to the "high" thread pool in order by rank; when the high thread pool is exhausted, the remainder will be run on the "low" pool
• A default rank can be assigned using the order of Worker creation, but an explicit rank can also be provided to avoid having to re-juggle code order when deciding on priority
• No rank needed or implied for "low" Workers
• Default value should be "low" to encourage use of E-cores and improve energy efficiency in default case

[mmccool]

More thoughts:

• If duplicate a rank, should cause an error
• If want to modify ranks after a worker has started... may cause a duplicate if done naively. Suggest adding a method to swap the ranks of two workers
• Should also be possible to assign a rank to a low worker, so if its priority is changed to high it can be taken into account

Some of the above relates to issue #5