Initial fun for Array iterator.

We are missing yet:

1. The special key+value return value (which I haven't seen in action anywhere yet, but still documented in the spec)
2. Plugging it into the Array @@iterator call.

I expected to send this request yesterday, but was too tired and forgot ;(

I don't think the kind is ever exposed to user code, is it?

I think instead of _ejs_atom_key and _ejs_atom_value, we can use EJS_ARRAYITER_KIND_KEY/EJS_ARRAYITER_KIND_VALUE