* Simplify postscript code parsing by using the `cvx` (convert to
executable) function.
* Tweak the test cases to match reality (returns list of stack operands,
or nil if stack is empty after exec).
- make sure readline has large enough string for steps2-A
- fix keyword? so that it doesn't leave anything extra on the stack
- fix argparse command line parsing so that that the second '--' on
the command line is not dropped.