Improved, extended, and refactored iteration special forms.

Improved, extended, and refactored iteration special forms.

 * Added support for DO*.

 * DO now does parallel assignment.

 * DO/DO* now correctly support init-forms such as:

     (do (a (b) (c 1) (d 1 (1+ d))) ...)

 * DO/DO*/DOTIMES/DOLIST/DOEACH now support return values via a lambda
   transform.

 * DOEACH now supports implicit key/value destructuring, e.g.:

     (doeach ((key val) arr res) ...)

 * Added labeled-for to facilitate creating labeled for loops.

 * Restructured low level printer operators to better match the
   underlying JS.