Make effector a stream to optimize enforcing speed by quick return

[GopherJ]

See also: casbin/casbin#439

My idea is to make effector a writable stream (Sink + Stream), once a policy has been analyzed, we write it into the stream and the stream will use different strategy to analyze the result.

Once the final result is produced, we return it as the result of e.enforce

[GopherJ]

Or maybe we should just implement it in enforce method?

[GopherJ]

The former makes it more general but harder to implement. The latter is simpler but less general (we need to put everything in enforce method, now it's ok we have only 4 effect strategies but how about having more?)

[hsluoyz]

We should extract the effector out. I think the idea about stream sounds good. Is sink + stream a built-in concept/operation in Rust?

[GopherJ]

@hsluoyz It's not a built in concept but pretty common. I didn't try to implement it because some potential problems need to be resolved but generally it should be feasible.

[GopherJ]

implemented in #142