Make symbols for keywords beforehand.

Before:
> R -q -e "source('Rlisp.R')" < ../zslisp/fib5.zsl  7.86s user 0.06s system 97% cpu 8.141 total
After:
> R -q -e "source('Rlisp.R')" < ../zslisp/fib5.zsl  7.20s user 0.06s system 97% cpu 7.430 total
Add simple loop syntax.