New version of js_events with semantics closer to Lwt_js_events (on top of PR #6)

[balat]

Experimental version of Js_events with a semantics closer to Lwt_js_events.

Example:

let c = clicks e h in
...
Lwt.cancel c

would become:

Eio_js.start @@ fun () ->
Eio.Switch.run @@ fun sw ->
Eio.Fiber.fork ~sw (fun () -> clicks e h);
...
Eio.Switch.fail sw Exn