Add an option to access current runnable (necessary for async/await project)

Author: vsilaev

net.tascalate.javaflow.api/src/main/java/org/apache/commons/javaflow/core/Stack.java

@@ -273,6 +273,11 @@ public boolean isSerializable() {
     public boolean isEmpty() {
         return iTop==0 && lTop==0 && dTop==0 && fTop==0 && oTop==0 && rTop==0;
     }
+    
+    public Runnable getRunnable() {
+        return runnable;
+    }
+    
 
     private String getStats() {
         final StringBuffer sb = new StringBuffer();