Fix memory leak in Tree implementation

- caused by retain cycle between parent and child (parent has a strong
  reference to child, and child has a strong reference to parent)

Author: chrisamanse

Tree/Tree.playground/Contents.swift

@@ -3,7 +3,7 @@
 public class TreeNode<T> {
   public var value: T
 
-  public var parent: TreeNode?
+  public weak var parent: TreeNode?
   public var children = [TreeNode<T>]()
 
   public init(value: T) {

Tree/Tree.swift

@@ -1,7 +1,7 @@
 public class TreeNode<T> {
   public var value: T
 
-  public var parent: TreeNode?
+  public weak var parent: TreeNode?
   public var children = [TreeNode<T>]()
 
   public init(value: T) {