Lisp: Change Lisp-to-JSON conventions.

Now a Lisp list is assumed to be an alist, and printed as a JSON
mapping; a JSON list is produced from a Lisp vector.  Also, `nil' is now
mapped to JSON `null'.

  * Implement these changes in `print-json-simple'.

  * Change the lists in `hosts.lisp' to be vectors.

  * Modify `host-defs.lisp' to handle vectors instead of lists.