feat: add nsdictionary helpers

ios/Helpers/DictionaryCodingKey.swift

@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is largely a copy of code from Swift.org open source project's
+// files JSONEncoder.swift and Codeable.swift.
+//
+// Unfortunately those files do not expose the internal _JSONEncoder and
+// _JSONDecoder classes, which are in fact dictionary encoder/decoders and
+// precisely what we want...
+//
+// The original code is copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+// Modifications and additional code here is copyright (c) 2018 Sam Deane, and
+// is licensed under the same terms.
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+internal struct DictionaryCodingKey : CodingKey {
+    public var stringValue: String
+    public var intValue: Int?
+
+    public init?(stringValue: String) {
+        self.stringValue = stringValue
+        self.intValue = nil
+    }
+
+    public init?(intValue: Int) {
+        self.stringValue = "\(intValue)"
+        self.intValue = intValue
+    }
+
+    public init(stringValue: String, intValue: Int?) {
+        self.stringValue = stringValue
+        self.intValue = intValue
+    }
+
+    internal init(index: Int) {
+        self.stringValue = "Index \(index)"
+        self.intValue = index
+    }
+
+    internal static let `super` = DictionaryCodingKey(stringValue: "super")!
+}