Improve the performance of the new BatchEnforce() API

We add a very raw BatchEnforce() API here: https://github.com/casbin/casbin/issues/617

It simply call `enforce()` in a for loop. The performance is still low if you want to list all the objects that you have access. Our next step is to do something like "set" operation, e.g., given all the objects as a set, input it into the matcher and calculate the subset that fulfills the matcher, then we have the allowed object list.