Improve Swift syntax (#25)

* Replace init-ializable struct with enums

* Add AnyObject requirement to protocols

* Hide init?(coder aDecoder: NSCoder) func

* Improve Swift syntax

* Revert "Replace init-ializable struct with enums"

This reverts commit 1be2e8c23cfc75959d7fd1c90028d1a7a257b7d1.

PanModal/Presentable/PanModalPresentable.swift

@@ -18,7 +18,7 @@ import UIKit
  }
  ```
  */
-public protocol PanModalPresentable {
+public protocol PanModalPresentable: AnyObject {
 
     /**
      The scroll view embedded in the view controller.

PanModal/Presenter/PanModalPresenter.swift

@@ -17,7 +17,7 @@ import UIKit
                                              sourceRect: .zero)
  ```
  */
-protocol PanModalPresenter {
+protocol PanModalPresenter: AnyObject {
 
     /**
      A flag that returns true if the current presented view controller

PanModal/View/PanContainerView.swift

@@ -20,8 +20,9 @@ class PanContainerView: UIView {
         addSubview(presentedView)
     }
 
+    @available(*, unavailable)
     required init?(coder aDecoder: NSCoder) {
-        fatalError()
+        fatalError("init(coder:) has not been implemented")
     }
 
 }
@@ -33,7 +34,9 @@ extension UIView {
      from the view hierachy
      */
     var panContainerView: PanContainerView? {
-        return subviews.compactMap({ $0 as? PanContainerView }).first
+        return subviews.first(where: { view -> Bool in
+            view is PanContainerView
+        }) as? PanContainerView
     }
 
 }