Move clock into IO.

benchmark/for.wren

@@ -1,10 +1,10 @@
 var list = []
 
-var start = OS.clock
+var start = IO.clock
 for (i in 0...1000000) list.add(i)
 
 var sum = 0
 for (i in list) sum = sum + i
 
 IO.print(sum)
-IO.print("elapsed: " + (OS.clock - start).toString)
+IO.print("elapsed: " + (IO.clock - start).toString)