transform + reduce async iterable cancellation

[richytong]

this issue names transducers as a workaround for early cancellation of async iterables. Transducer enabled functions reduce and transform are in a great position to cancel async iteration because they control the for await...of loop that started the async iteration

async iterable cancellation is desirable for example in front-end applications where you may be streaming data at a time and when the user navigates away you should not care about streaming data anymore and cancel at that point.

I like this API

const myOngoingTaskPromise = transform(f, null)(myInfiniteSource)
myOngoingTaskPromise.cancel() // like Bluebird

[richytong]

nice one