Use TaskGroups instead of array of tasks

[Frizlab]

Rationale for this comes from this thread https://forums.swift.org/t/taskgroup-vs-an-array-of-tasks/53931
Apparently launching a lot of Tasks just like that is not very good, and we should use task groups instead.

I did not test the performance implication of this change, though. Maybe it’s actually worse because of the additional sort needed. In theory it should be better though.

[AndrewBarba]

Instead of sorting does it make sense to do something like:

var results = Array<T?>(repeating: nil, count: self.count)
...
for try await result in group {
    results[result.offset] = result.value
}
return results as! [T]

[Frizlab]

@AndrewBarba I’d say yes. And it’s probably faster too. I’ll push your alternative.

[Frizlab]

Done. However, count is not available in a generic Sequence, so I have to grow the result array as I retrieve the results. I’m not sure we actually gain anything compared to the sort solution. We should measure both solution to find the fastest.
Previous implementation is still available here 1105df4.

[tachyonics]

I don't think it is necessary to have count available on the input Sequence. We are iterating through the sequence to add tasks to the group so we know how many tasks we have added to the group and therefore how many we are expecting back-

https://github.com/tachyonics/CollectionConcurrencyKit/blob/main/Sources/CollectionConcurrencyKit.swift#L123

[Frizlab]

@tachyonics You’re not wrong. I have pushed your method.

[johnrogers]

Fully support these changes. Not only will it make the functions more performant, but it will ensure each function participates in Task cancellation inheritance correctly.