Support the ES6 iteration protocol

[rauschma]

Among other changes: If it is there, you should use Symbol.iterator as a property key.

Long-term, it may also make sense to rename Iterator, in order to avoid confusion with the built-in API.

[Maher4Ever]

Adding this to a specialized Set I have was as easy as (other code omitted for brevity):

import {Set as CollectionsSet} from 'collections/set'

class ParametersSet extends CollectionsSet {
  [Symbol.iterator]() {
    return this.iterate()
  }
}

It would be awesome to have this natively implemented in all collections though.