export PDispatcher.handles
Hi, I'm working on porting `httpbeast` to windows and also porting `httpbeast` to `chronos`, so I need to register handles in `PDispatcher`.

As shown below, I think `PDispatcher.handles` should be exported.
https://github.com/nim-lang/Nim/pull/15140

I have benchmarked `httpbeast` based on `asyncdispatch` and `chronos` in windows(based on IOCP) with 
600 threads, 1s ramp-up period, 1000 loop count using jmeter. Of course I have not modified `httpbeast` to accommodate `chronos`.

asyncdispatch results:
![asyncdispatch](https://user-images.githubusercontent.com/43030857/89615429-2df80600-d8b9-11ea-95c9-a1602d7c27b5.png)

chronos results:
![chronos](https://user-images.githubusercontent.com/43030857/89616120-621ff680-d8ba-11ea-9b1f-e0c84f9e78a3.jpg)

Test program:

https://gist.github.com/xflywind/9782cb906be3f83d3a3a3cc9c85debae
