[Foundation] Update CharacterSet and NSCharacterSet punctuationCharacters to more consistent naming

Addresses rdar://problem/26628434 Swift3: "whitespaces"?

Author: phausler

apinotes/Foundation.apinotes

@@ -44,6 +44,8 @@ Classes:
     SwiftName: symbols
   - Name: newlineCharacterSet
     SwiftName: newlines
+  - Name: punctuationCharacterSet
+    SwiftName: punctuationCharacters
   Methods:
   - Selector: 'hasMemberInPlane:'
     SwiftName: hasMemberInPlane(_:)

stdlib/public/SDK/Foundation/CharacterSet.swift

@@ -202,9 +202,14 @@ public struct CharacterSet : ReferenceConvertible, Equatable, Hashable, SetAlgeb
         return CharacterSet(reference: NSCharacterSet.illegalCharacters as NSCharacterSet)
     }
 
-    /// Returns a character set containing the characters in Unicode General Category P*.
+    @available(*, unavailable, renamed: "punctuationCharacters")
     public static var punctuation : CharacterSet {
-        return CharacterSet(reference: NSCharacterSet.punctuation as NSCharacterSet)
+        return CharacterSet(reference: NSCharacterSet.punctuationCharacters as NSCharacterSet)
+    }
+
+    /// Returns a character set containing the characters in Unicode General Category P*.
+    public static var punctuationCharacters : CharacterSet {
+        return CharacterSet(reference: NSCharacterSet.punctuationCharacters as NSCharacterSet)
     }
 
     /// Returns a character set containing the characters in Unicode General Category Lt.