Add `Disposable` concept

[dalu93]

To workaround the memory leak issue, an another idea could be to implement the famous pattern of Disposable and Disposer

// MARK: - Disposable declaration
public protocol Disposable {
    func dispose()
}

// MARK: - Disposable
extension Each: Disposable {
    public func dispose() {
        stop()
    }
}

// MARK: - Disposer declaration
public protocol Disposer {
    func add(_ disposable: Disposable)
    func dispose()
}