`ReplaySubject` and `.Subscription` scaffolding.

replay_subject_and_subscription_scaffolding.swift

final class ReplaySubject<Output, Failure: Error>: Subject {
	typealias Output = Output
	typealias Failure = Failure

	private let bufferSize: Int

	init(bufferSize: Int) {
		self.bufferSize = bufferSize
	}
	
	func send(subscription: Combine.Subscription) { /// (1) We’ll need to fully qualify `Combine.Subscription` to
	/// make it clear to the compiler–if this extra step isn’t your cup of tea, it’s also totally valid to name the
	/// types something else.
		/* … */
	}
	
	func send(completion: Subscribers.Completion<Failure>) { /* … */ }

	func send(_ value: Output) { /* … */ }

	func receive<Subscriber: Combine.Subscriber>(
		subscriber: Subscriber
	) where Failure == Subscriber.Failure, Output == Subscriber.Input { /* … */ }
}

extension ReplaySubject {
	final class Subscription: Combine.Subscription { /// (2) Same note as `(1)`.
		func request(_ demand: Subscribers.Demand) { /* … */ }

		func cancel() { /* … */ }
	}
}