I've pushed the SBCL closure size reduction.

I'll be the road for a while, unable to work on this patch, so if anyone
wants to work on it and merge, please go ahead :)

Left to do:

- Suggestion: add a keyword to choose between asdf:compile-system and
  asdf:load-system (default should be asdf:load-system).
- Make sure sbcl-stumpwm-kbd-layouts usees asdf:compile-system.
- Rebuild the Lisp world to test.

Cheers!
Pierre