add asyncForEach

[quadramadery]

I've been playing around with Issue #17

Example asyncForEach, using async/await. If you like it, I can add asyncRange as well.
Although asyncForEach fails miserably in benchmark.

[w8r]

Thanks a lot! I will look into it (try and find how to compile it into es5-6). Do you have some use cases in mind?

[vladislav-atakhanov]

I think it's better to add an [Symbol.iterator]. Iterators are used in more cases than asyncForEach.

*[Symbol.iterator]() {
  var current = this._root
  var stack = []

  var done = false
  while (!done) {
    if (current) {
      stack.push(current)
      current = current.left
    } else {
      if (stack.length > 0) {
        current = stack.pop()
        yield current
        current = current.right
      } else done = true
    }
  }
}