Initial Chicken Scheme Support

If you are able, could you please give it a smoke test before merging?

I've given it a look over myself; but I must admit I'm quite weary from the late, short nights with my newborn, and I am not totally familiar with Geiser's full functionality.
since chicken doesn't have a special debugger prompt a la guile, the correct value for this variable is `nil`.  as a side-effect, that will solve the problem with the repl history not being saved (by default, we don't keep a history of debugging prompts, and right now geiser thinks that the repl is always in debugging mode).