Derive the hash code from the object id, which is more efficient than…

… the default identity hash code. Also clean up the string representation.

src/main/java/org/cojen/dirmi/core/Stub.java

@@ -34,19 +34,25 @@ protected Stub(long id) {
     abstract StubInvoker invoker();
 
     @Override
-    public String toString() {
+    public final int hashCode() {
+        return Long.hashCode(id);
+    }
+
+    @Override
+    public final String toString() {
+        var b = new StringBuilder();
+
         String name = getClass().getName();
 
-        // Prune off the generated suffix.
         int ix = name.lastIndexOf('-');
-        if (ix > 0) {
-            name = name.substring(0, ix);
+        if (ix < 0) {
+            b.append(name);
+        } else {
+            // Prune off the generated suffix.
+            b.append(name, 0, ix);
         }
 
-        var b = new StringBuilder();
-
-        b.append(name).append('@').append(Integer.toHexString(System.identityHashCode(this)))
-            .append("{id=").append(id);
+        b.append("{id=").append(id);
 
         support().appendInfo(b);