Add Swift refinements for TreeElement

Author: jcmosc

Sources/OpenGraph/Tree/TreeElement.swift

@@ -0,0 +1,28 @@
+//
+//  TreeElement.swift
+//  OpenGraph
+
+public import OpenGraphCxx
+
+extension TreeElement {
+    public var value: AnyAttribute? {
+        let result = __OGTreeElementGetValue(self)
+        return result == .nil ? nil : result
+    }
+}
+
+extension Nodes: @retroactive IteratorProtocol {
+    public typealias Element = AnyAttribute
+    public mutating func next() -> AnyAttribute? {
+        let result = __OGTreeElementGetNextNode(&self)
+        return result == .nil ? nil : result
+    }
+}
+
+extension Children: @retroactive IteratorProtocol {
+    public typealias Element = TreeElement
+}
+
+extension Values: @retroactive IteratorProtocol {
+    public typealias Element = TreeValue
+}