Make the underscore wrapper iterable

[jdalton]

Thoughts on making the Underscore wrapper an iterator and iterable?

It would require detecting Symbol.iterator and doing something like

_.prototype[Symbol.iterator] = function() {
  return this;
};
_.prototype.next = function() {
  if (this.__values__ === undefined) {
    this.__values__ = _.toArray(this.value());
  }
  var done = this.__index__ >= this.__values__.length,
      value = done ? undefined : this.__values__[this.__index__++];
  return { 'done': done, 'value': value };
};

Related to jashkenas/backbone#3954 (comment).

[jgonggrijp]

Should only be implemented if #2147 is implemented, too. In addition, I disagree with the above code example as it would be possible to iterate the wrapper only once, despite the fact that the underlying values are not really consumed.