Add immutable Map to stdlib

Author: odersky

tests/pos-special/stdlib/collection/immutable/HashMap.scala

@@ -386,7 +386,7 @@ final class HashMap[K, +V] private[immutable] (private[immutable] val rootNode:
     else new HashMap(newRootNode)
   }
 
-  override def removedAll(keys: IterableOnce[K]): HashMap[K, V] = {
+  override def removedAll(keys: IterableOnce[K]^): HashMap[K, V] = {
     if (isEmpty) {
       this
     } else {