Fix Event.write to have writable = true

[edwintorok]

I was testing the library and my code got "stuck", I tried using Poll.set mux socket Poll.Event.write to handle EINPROGRESS from connect, but strace was showing that the FD was not getting added to the epoll set.
This was because the code thought no change was requested, since Event.write was equal to Event.none.

[anuragsoni]

This was an embarrassing miss on my part!!

Thank you!

[edwintorok]

Thanks for merging, there are still some other problems, e.g. 'oneshot' seems to be added to each event, so now after one event everything is still stuck.... (unless I keep changing epoll set)

[anuragsoni]

I’ve been thinking about allowing users to specify one shot vs level triggered events. Both kqueue and epoll support it so this seems like an option worth surfacing. I liked the idea of one shot events so a readiness event is only reported once instead of multiple times, and then considering file descriptors always ready unless encountering EAGAIN

[edwintorok]

Giving control the user sounds right, I have no experience with kqueue though, so I"ll leave that improvement idea to you.