Implement ClosedSet as an actual set

[meiamsome]

Recent versions of ECMAScript include a Set type: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set

Indeed, they are required to be implemented in sub-linear time using hash tables or similar.

[shiffman]

Is this Set only ES6? Really struggling to sort out when I should examine and discuss ES6 topics as part of my videos.