Add parse built-in

[ngeiswei]

Fix for #109.

[ngeiswei]

Needless to say you should carefully check because I am not an experienced Prolog programmer.

Also, I don't know if the test

!(test (parse "(* 2 21)") (quote (* 2 21)))

is making too much commitment about both parse and quote. Let me know if you wish me to remove it, or to add other tests, for that matter.

[patham9]

Thank you very much Nil! :)