Make @threads work on array comprehensions

[IanButterworth]

Closes #209

See docstring for examples.

Developed with help from Claude.

[adienes]

the fact that this always returns a Vector{Any} is a bit rough

just some further cases that might want to be handled:

multiple loops

julia> @threads [i + j for i in 1:3, j in 1:3]
ERROR: TaskFailedException

non-indexable iterators

julia> @threads [i for i in Iterators.flatten(1:3)]
ERROR: TaskFailedException

[ericphanson]

it seems a little odd to me that :greedy does not preserve order. It seems mostly the different flavors change performance characteristics without changing semantics (or only changing semantics when you rely on details like threadids), but unordered is a pretty big semantic change.

Could it be called :unordered or :greedy_unordered or something to make it explicit? Or alternatively, could we just order it post-hoc by passing the index along?

(BTW: feels like there might be some similarity to Threads.foreach(f, c::Channel) from #34543 which I think is still not a well-known feature)

[IanButterworth]

If Threads.foreach(f, c::Channel) could gain a filter kwarg, it might be worth just making this a wrapper for that?

[IanButterworth]

@adienes I addressed your points in #59019 (comment)

@ericphanson I kept :greedy to match the existing @threads option. Not strongly attached, but it seems simplest to keep it the same.

[Seelengrab]

These two tests seem wrong. :greedy does not guarantee any particular execution order, so there is no reason it should be preserved. It's better to check that every element in the result is also present when the same kind of collection is done without threading:

Suggested change

	@test all(result_greedy[i] == i^2 for i in 1:n)
	@test issorted(result_greedy) # should be ordered for greedy scheduling too
	result_nonthreaded = [ i^2 for i in 1:n ]
	@test all(result_greedy) do r
	r in result_nonthreaded
	end

This is of course quadratic, but since :greedy doesn't guarantee an order, anything else runs into ordering problems.

[IanButterworth]

But now we store i greedy has been made order-preserving via the sort at the end.

[Seelengrab]

:greedy should not have to guarantee that though. The suggestion for adding the original index I gave on slack was for :static and :dynamic, which are order preserving by design.

[Seelengrab]

Same as above, :greedy does not guarantee this.

[Seelengrab]

Same as above.