Renamed each -> forEach.
Made s.filter the method called by s.call.
Improved s.forEach to allow a `this` argument (now `s.forEach(console.log, console)` works).
Added set.toArray.
Made s.add use set.add instead of set.push.  Also made empty s.map return an empty set.
Made all set-related functions use set.add instead of set.push. Made set.push an alias to set.add, for maintaining uniqueness.
Patched an API-change problem.
Allowed s.size to be accessed through s.get.
Improved s.toString.
Got s.toString working with keys.
Implemented set.symmetricDifference.