Change _downKeys from Array to Object

Submited these to https://github.com/limikael/p5.js/pull/1 but @limikael had no time to analyze and commit them!
Changed `Array` to `Object` as @hamoid advised @ https://github.com/lmccart/p5.js/pull/411#issuecomment-64721647.
Kept `Array` & `Set` commented out so it's easier to toggle among any of them in the future!  ^_^