Add `next(isolation:) async throws(Failure)` support to async sequences

[alex-waldron]

SE-0421: Generalize effect polymorphism for AsyncSequence and AsyncIteratorProtocol introduced next(isolation:) to AsyncIteratorProtocol. Poking around this package, I realized that most iterators are yet to implement the isolated version of next. Is there a specific reason why?

[alex-waldron]

For a bit more context on how I stumbled across this ...

I am using adjacentPairs to get previous and current values from an Observations async sequence. I ran into some issues when consuming the sequence within an asynchronous context spun up using Task.immediate. I expected that if I did something like

@Observable class TestObservable {
  var prop = 1
}

let obj = TestObservable()
let props = Observations { obj.prop }
Task.immediate {
    for await (previous, current) in props.adjacentPairs() {
        print(previous, current) // expect "1 2" but nothing was printed
    }
}
obj.prop = 2

"1 2" would be printed. Instead, I got nothing. The root cause was the missing implementation of next(isolation:) in adjacentPairs. I opened up #390 to fix it but it felt odd since no other "AsyncAlgorithms 1.0" sequences have done this. I also couldn't find any docs on plans to add this support. Is the work planned? Would a change like this require a major version bump since it changes behavior? If you all think this is worth pursuing, I'd be happy to help implement it across the other sequences.

[alex-waldron]

Another datapoint: you can get a crash at run time in merge, chain, and potentially others when combining a throwing and a non throwing async sequence.

let nums = [1,2,3].async
let throwingNums = [4,5,6].async.map { i in
    if i == 5 {
        throw NSError(domain: "test", code: 999)
    } else {
        return i
    }
}

let chained = chain(nums, throwingNums)

for await _ in chained {} // ❌ Crash: Could not cast value of type 'NSError' (0xa72a6f538) to 'Swift.Never' (0x202e3b8b0)