new LinkedHashMap/LinkedHashSet implementation

The mutable HashMap/HashSet has been rewroten and the
performance is better. So rewrote the LinkedHashMap and
LinkedHashSet also to improve performance. The detailed
data can be seen in the PR.
Most codes are same with HashMap/HashSet but some are
different:
1. the stepper is not overridden since order may be broken.
2. specific method to handle the order when adding/removing
the nodes.
3. other minor changes.

Signed-off-by: Liang Yan <ckgppl_yan@sina.cn>

src/library/scala/collection/immutable/ListMap.scala

@@ -246,7 +246,7 @@ object ListMap extends MapFactory[ListMap] {
         // by directly iterating through LinkedHashMap entries, we save creating intermediate tuples for each
         // key-value pair
         var current: ListMap[K, V] = empty[K, V]
-        var firstEntry = lhm._firstEntry
+        var firstEntry = lhm._firstNode
         while (firstEntry ne null) {
           current = new Node(firstEntry.key, firstEntry.value, current)
           firstEntry = firstEntry.later
@@ -349,7 +349,7 @@ private[immutable] final class ListMapBuilder[K, V] extends mutable.ReusableBuil
     } else xs match {
       case lhm: collection.mutable.LinkedHashMap[K, V] =>
         // special-casing LinkedHashMap avoids creating of Iterator and tuples for each key-value
-        var firstEntry = lhm._firstEntry
+        var firstEntry = lhm._firstNode
         while (firstEntry ne null) {
           underlying = new ListMap.Node(firstEntry.key, firstEntry.value, underlying)
           firstEntry = firstEntry.later