[RxCocoa] Use `bind(to:)` on an array of `ObserverType`

Array+ObserverType.swift

extension Array: ObserverType where Element: ObserverType {
    public typealias E = Element.E

    public func on(_ event: Event<E>) {
        forEach { (element) in
            element.on(event)
        }
    }
}

extension Array where Element: ObserverType {
    func asObserver() -> AnyObserver<Element.Element> {
        return AnyObserver<Element.Element>(eventHandler: { (event) in
            self.forEach { (element) in
                element.on(event)
            }
        })
    }
}