Implement lazyMap as an event transformation.

[andersio]

So that it abides to the interruption on specified scheduler rule exhibited by its peer async operators.

Checklist

• Updated CHANGELOG.md.

[NachoSoto]

Does this fix a bug? If so it might be worth adding a test. Otherwise doesn't seem worth rewriting the operator from scratch…

[andersio]

@NachoSoto Done in c8e3a8c.

The majority of async operators forward all events to the specified scheduler, and the downstream initiated interrupted event has been respecting the specified closest scheduler in the path* since 2.0. So the lazyMap here is aligned to them for consistency.

* Edit: So that one won't get a surprise of interrupted not being delivered on the designated scheduler, in the case of an observer accessing queue confined resources. e.g.

let disposable = producer
    .observe(on: scheduler)
    .start(observerAccessingResourcesConfinedToTheSchedulerAbove)

DispatchQueue.global(qos: .userInitiative).asyncAfter(wallTime: .now() + .seconds(10)) {
    disposable.disposed() // In 1.0 & earlier, this would cause an immediate
                 // `interrupted` event on this global concurrent queue.
}