Is your feature request related to a problem? Please describe.
The existing SwifterSwift extension MKPolyline.coordinates uses method getCoordinates that is defined in MKMultiPoint and not in MKPolyline.
MKPolyline extends MKMultiPoint.
The negative effect is that this extension is not available in all subclasses of MKMultiPoint where this extension makes sense, namely MKPolygon

Describe the solution you'd like
Instead of
public extension MKPolyline {
var coordinates: [CLLocationCoordinate2D] {
write
public extension MKMultiPoint {
var coordinates: [CLLocationCoordinate2D] {

This makes coordinates available in MKPolyline and in MKPolygon

Feature was introduced in #416

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