Expose execute function to Objective-C API's

[basememara]

Allows encapsulation of debounce logic in lazy loaded properties and feed into legacy Objective-C API's like NotificationCenter, see #selector(refreshProgress.execute):

class ViewController: UIViewController {
	
    lazy var refreshProgress: DebouncedLimiter = {
        return DebouncedLimiter(limit: 3) { [weak self] in
            self?.debouncedExecutionCount += 1
        }
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        NotificationCenter.default.addObserver(refreshProgress,
            selector: #selector(refreshProgress.execute),
            name: .UIApplicationDidBecomeActive,
            object: nil
        )
    }
}