[Feature] storage for collection view controller

[royhsu]

protocol Model: Unique {

    associatedtype IDKey

    var idKey: KeyPath<Model, IDKey> { get }

}

enum StorageValueEvent {

    case new(value: Value)

    case updated(value: Value)

    case failed(value: Value, error: Error)

}

protocol Storage {

    associdatetype: Value: Model

    func observe(
        _ observer: (Result<[StorageValueEvent]> -> Void)
    )

    func update(_ value: Value)

}