tail recursion for interpreter.py

[natethern]

Got the idea from
https://www.norvig.com/lispy.html
https://www.norvig.com/lispy2.html

Create long-recursion.scm in examples:

(define (odd? n)
  (if (<= n 0)
      #f
      (even? (- n 1))))
(define (even? n)
  (if (<= n 0)
      #t
      (odd? (- n 1))))
(write (even? (* 7 7 7 7 7 7 7)))

python compile_file.py long-recursion.scm && python run_compiled.py long-recursion.bobc ==> #f

python interpret_file.py long-recursion.scm will segfault before the patch
python interpret_file.py long-recursion.scm ==> #f after the patch

[natethern]

Cleaned-up version of my previous request with improved comments and docstrings.

[eliben]

Thanks again, this looks pretty good!

As mentioned before, I will leave this PR open for now. I have to think more whether I want to keep interpreter.py as simple as possible, leaving the optimizations to the other methods of running bobscheme (VMs, etc)