modified to use irep->reps to reference child ireps. preparation for

removing irep array from mrb_state.  note that instructions OP_LAMBDA,
OP_EXEC and OP_EPUSH are incompatible, and dumped mrb format has changed.

src/parse.y

@@ -5378,7 +5378,7 @@ load_exec(mrb_state *mrb, parser_state *p, mrbc_context *c)
   }
   if (c) {
     if (c->dump_result) codedump_all(mrb, proc);
-    if (c->no_exec) return mrb_fixnum_value(0);
+    if (c->no_exec) return mrb_obj_value(proc);
     if (c->target_class) {
       target = c->target_class;
     }