Merge remote-tracking branch 'origin' into swift-3-api-guidelines

stdlib/public/core/StringUnicodeScalarView.swift

@@ -44,7 +44,7 @@ extension String {
       }
       mutating func next() -> UTF16.CodeUnit? {
         if idx == core.endIndex {
-          return .None
+          return nil
         }
         return self.core[idx++]
       }
@@ -174,7 +174,7 @@ extension String {
             switch self._asciiBase.next() {
             case let x?:
               result = .Result(UnicodeScalar(x))
-            case .None:
+            case nil:
               result = .EmptyInput
             }
           } else {
@@ -187,7 +187,7 @@ extension String {
         case .Result(let us):
           return us
         case .EmptyInput:
-          return .None
+          return nil
         case .Error:
           return UnicodeScalar(0xfffd)
         }