for-of example should use array destructuring to group-assign without a temp

[BrendanEich]

Instead of

var fibonacci = {
  [Symbol.iterator]() {
    var pre = 0, cur = 1;
    return {
      next() {
        var temp = pre; pre = cur; cur += temp;
        return { done: false, value: cur }
      }
    }
  }
}

use this:

let fibonacci = {
  [Symbol.iterator]() {
    let pre = 0, cur = 1;
    return {
      next() {
        [pre, cur] = [cur, pre + cur];
        return { done: false, value: cur }
      }
    }
  }
}

I threw in let usage over var for free :-P.

/be