`Publisher.materialize` with finished event handling.

materialize_finished.swift

extension Publisher {
	func materialize() -> AnyPublisher<Event<Output, Failure>, Never> {
		map(Event.value)
			.append(.completion(.finished)) /// (1) Intercept an upstream finished event
			/// and emit an `Event.completion(.finished)` (and then send down an
			/// ordinary finished event).

		/* … */
	}
}