bug fixed & more efficient callfv

I changed callfv's way of calling a function with arguments in vm_vec.
now callfv fetches arguments from val_stack directly,so it runs test/fib.nas from 2.4s to 1.9s.

delete operand callf,add operands callfv & callfh.

also,i check val_stack's top to make sure there is not a stack overflow.

lib.nas

@@ -26,8 +26,7 @@ var setsize=func(vector,size)
 }
 var system=func(str)
 {
-    __builtin_system(str);
-    return;
+    return __builtin_system(str);
 }
 var input=func()
 {