Leaked events, cause Maximum call stack with Mouse Position behavior

[hgiasac]

Since version 7.0, native events have been replaced by FRP Event, which is somewhat too heavy weight for continuous ones such as mouse position event.

Let's begin with buttons example:
http://try.purescript.org/?backend=behaviors&gist=53e36cc4956cc67e684dd6c48b778385

Before version 7.0, we can render more than 100 buttons without lagging. Now the performance is every bad with 6 buttons, and cause Maximum call stack error with 10 buttons.

From what I check in debug log, the issue may be in traversable
buttons = traverse toButton (0..9)
In expectation, each button is subscribed once, 10 buttons produce 10 behavior events. But in debug, the more buttons are created, the more duplicated subscriptions. With 5 buttons, there are about 100 created events. It is the nightmare because of resource leaking

I don't know if this is a bug, or there is another solution for this.