Small fix.
Make symbols for keywords beforehand.

Before:
>  python pylisp.py < ../zslisp/car_rec.zsl  3.44s user 0.02s system 93% cpu 3.696 total
After:
> python pylisp.py < ../zslisp/car_rec.zsl  3.05s user 0.02s system 92% cpu 3.311 total
Add simple loop syntax.

> (defun f(n a) (loop (if (eq n 0) (return a)) (setq a (* a n)) (setq n (- n 1))))
f
> (f 10 1)
3628800
Set recursionlimit.