fixed minor bug on stack pop/push
Evaluator.eval_quoted now accepts temp_vars

* optional argument, defaults to []
* temp_vars are added to the binding before eval and removed from
  the binding after the evaluation
* in case of name collisions, variables previously declared prevail
* untreated corner case: names defined withing block of code and
  added as temp_vars aren't treated
* used for evaluating exceptions, as they can't be added to the
  syntax tree.
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