`ReplaySubject` start.

replay_subject_sketch.swift

final class ReplaySubject: Subject {
	typealias Output = /* … */ /// (1) The `Subject` itself won’t know its `Output` and `Failure` types,
	/// but callers will. So, we can add generics for the two, following `PassthroughSubject` and
	/// `CurrentValueSubject`’s lead.

	typealias Failure = /* … */ /// (2) Same as in `(1)`.

	private let bufferSize: Int

	init(bufferSize: Int) {
		self.bufferSize = bufferSize
	}
	
	func send(subscription: Subscription) { /* … */ }
	
	func send(completion: Subscribers.Completion<Failure>) { /* … */ }

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