Small fix.
Use class instead of Hash.

Before:
> ruby rblisp.rb < ../zslisp/car_rec.zsl  10.09s user 0.03s system 96% cpu 10.439 total
After:
> ruby rblisp.rb < ../zslisp/car_rec.zsl  2.02s user 0.03s system 97% cpu 2.088 total
Make symbols for keywords beforehand.

Before:
> ruby rblisp.rb < ../zslisp/car_rec.zsl  2.02s user 0.03s system 97% cpu 2.088 total
After:
> ruby rblisp.rb < ../zslisp/car_rec.zsl  1.52s user 0.02s system 98% cpu 1.565 total
Add simple loop syntax.