Fixed retain cycles between MacawView and MacawZoom

In this pull request I fixed 2 issues:

retain cycle between MacawView and MacawZoom: both classes referenced to each other using strong reference which produced retain cycle previously. Solution is easy - just to use some weak / unowned reference.
implicit retain cycle with onZoomChange method. As we know - self is also implicitly captured when we pass method as closure. So instead of directly passing method name as closure - zoom.initialize(view: self, onChange: onZoomChange) we have to wrap it into additional closure with [weak self] parameter in capture list:

zoom.initialize(onChange: { [weak self] transform in
    self?.onZoomChange(t: transform)
})

exyte / Macaw