This will not teach you much about how to implement real Forth. The most interesting part is probably
how one can keep references to Pony functions (the Forth words) in a Map (the _dict).
Without the crucial help I got from Chris and Joe, I would have had problems solving that part.
I more or less ported the Scala code found at https://github.com/joneshf/forla.
The readline part is based on the Pony readline example.
There may of course be things in my Pony code that could be improved. Feel free to file an issue.