Implement debug() using LinkedHashSet for determinism

In general, we should use LinkedHashSet/LinkedHashMap for collections
that we iterate over; this makes the behavior deterministic.  For
collections that are merely accessed but not iterated over, HashSet
etc. are fine.

src/main/scala/Module.scala

@@ -30,12 +30,9 @@
 
 package Chisel
 import scala.math._
-import scala.collection.mutable.ArrayBuffer
+import scala.collection.mutable.{ArrayBuffer, Stack, BitSet}
+import scala.collection.mutable.{LinkedHashSet, HashSet, HashMap}
 import scala.collection.mutable.{Queue=>ScalaQueue}
-import scala.collection.mutable.Stack
-import scala.collection.mutable.HashSet
-import scala.collection.mutable.HashMap
-import scala.collection.mutable.BitSet
 import java.lang.reflect.Modifier._
 import scala.sys.process._
 import scala.math.max;
@@ -268,7 +265,7 @@ abstract class Module(var clock: Clock = null, private var _reset: Bool = null)
   var parent: Module = null;
   var containsReg = false;
   val children = ArrayBuffer[Module]()
-  val debugs = HashSet[Node]()
+  val debugs = LinkedHashSet[Node]()
   val printfs = ArrayBuffer[Printf]()
   val asserts = ArrayBuffer[Assert]()