These aren't strictly necessary given a bufferAhead helper, because you can always replace .some(pred) with .map(pred).bufferAhead(3).some(x => x) (or similar). But that's kind of gross.

I definitely don't want to add a "concurrency" parameter to the iterator-producing helpers, since it's much easier to use when concurrency of an iterator chain is determined by how many times the consumer pulls from it. (Some more discussion of that in the second half of this comment.)

But these methods are the consumer, and might themselves be async and able to run their functions concurrently. Should we have concurrent versions of the consumers?