Set.any should be a method

Although Set.any will always return the same element when not modifying
the set, that is an implementation detail. It should be a method.

Swift SVN r23697

stdlib/core/HashedCollections.swift.gyb

@@ -394,7 +394,7 @@ public struct _Set<T : Hashable> :
   /// Remove a member from the set and return it if `count` > 0,
   /// else return `nil`.
   public mutating func removeAny() -> T? {
-    return (count > 0) ? remove(any!) : nil
+    return (count > 0) ? remove(any()!) : nil
   }
 
   /// The number of members in the set.
@@ -471,7 +471,7 @@ public struct _Set<T : Hashable> :
   }
 
   /// Return any member of the set if `count` > 0, else return `nil`.
-  public var any: T? {
+  public func any() -> T? {
     return count > 0 ? self[startIndex] : .None
   }