some performance improvements

[klapauciusisgreat]

https://github.com/vygr/OSX-Forth seems another nice forth that started out with jonesforth.

It may be interesting to consider the improved dictionary code there (looks like a simple hashtable) for jonesforth. I would only do that once I understand more about forth performance - this looks like a nice improvement, but it may not really buy that much performance and complicate/detract from the jonesforth tutorial.