Adds missing Immutable annotations.

Author: brianburton

src/main/java/org/javimmutable/collections/hash/map/MapEmptyNode.java

@@ -48,13 +48,14 @@
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
 
+@Immutable
 public class MapEmptyNode<K, V>
     implements MapNode<K, V>
 {
     private static final MapEmptyNode EMPTY = new MapEmptyNode();
 
-
     @SuppressWarnings("unchecked")
     public static <K, V> MapNode<K, V> of()
     {

src/main/java/org/javimmutable/collections/hash/map/MapMultiKeyLeafNode.java

@@ -48,12 +48,14 @@
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
 
 /**
  * HamtNode that stores only one value.  Any assign that would progress down the tree
  * below this node replaces it with a normal node instead.  These exist to shorten the
  * height of the overall tree structure when hashCodes are dispersed.
  */
+@Immutable
 public class MapMultiKeyLeafNode<K, V>
     implements MapNode<K, V>
 {

src/main/java/org/javimmutable/collections/hash/map/MapSingleKeyLeafNode.java

@@ -48,6 +48,7 @@
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
 import java.util.Objects;
 
 import static org.javimmutable.collections.MapEntry.entry;
@@ -57,6 +58,7 @@
  * below this node replaces it with a branch node instead.  These exist to shorten the
  * height of the overall tree structure when hashCodes are dispersed.
  */
+@Immutable
 public class MapSingleKeyLeafNode<K, V>
     implements MapNode<K, V>
 {

src/main/java/org/javimmutable/collections/hash/set/SetEmptyNode.java

@@ -44,7 +44,9 @@
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
 
+@Immutable
 public class SetEmptyNode<T>
     implements SetNode<T>
 {

src/main/java/org/javimmutable/collections/hash/set/SetMultiValueLeafNode.java

@@ -44,12 +44,14 @@
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
 
 /**
  * HamtNode that stores only one value.  Any assign that would progress down the tree
  * below this node replaces it with a normal node instead.  These exist to shorten the
  * height of the overall tree structure when hashCodes are dispersed.
  */
+@Immutable
 public class SetMultiValueLeafNode<T>
     implements SetNode<T>
 {

src/main/java/org/javimmutable/collections/hash/set/SetSingleValueLeafNode.java

@@ -44,13 +44,15 @@
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
 import java.util.Objects;
 
 /**
  * SetNode that stores only one value.  Any assign that would progress down the tree
  * below this node replaces it with a branch node instead.  These exist to shorten the
  * height of the overall tree structure when hashCodes are dispersed.
  */
+@Immutable
 public class SetSingleValueLeafNode<T>
     implements SetNode<T>
 {