Async TTM: Process input tokens via workstack in transformTokens.

* workStack is initialized to input tokens.
* Track elt-index on the stack array.
* Simplifies code by not having two separate paths for
  fetching tokens.
* No change in parser test results.
* Verified errorless RT-ing of en:Barack Obama.

Change-Id: Ie5a2231f1fbebc52e1a3b7ef424150486c4cc8fb