add Continuations test

test/extra/Continuations.java

@@ -0,0 +1,19 @@
+package extra;
+
+import static avian.Continuations.callWithCurrentContinuation;
+
+import avian.CallbackReceiver;
+import avian.Callback;
+
+public class Continuations {
+  public static void main(String[] args) throws Exception {
+    System.out.println
+      ("result: " +
+       callWithCurrentContinuation(new CallbackReceiver<Integer>() {
+           public Integer receive(Callback<Integer> continuation) {
+             continuation.handleResult(42);
+             throw new RuntimeException();
+           }
+         }));
+  }
+}