Add a sketch of a Python-based DSL

- Mostly for my future reference.  This doesn't look half bad, but I
  find the implementation involved (and also fairly hacky, what with
  overloading all the Python builtins).  I probably wouldn't want to
  debug a larger, more complete suite based on this sketch.  It's
  instructive though, and I don't want to throw it away, hence this
  commit.

- Clearly even with a language that's mostly concatenative (the DSL
  pretty much is), infix operators seem to be a good fit.  We might
  want to revisit that part of the actual language.  ?eq etc. are a
  good fit for the language, but I still find them somewhat awkward
  to type, even after being exposed to them for some months.