Hey Guix!

Here's another patch! This one adds a new Common Lisp 
implementation: ABCL on the JVM.

When I added Kawa we ended up deciding to put it in its own 
package (in kawa.scm). I wasn't sure whether it was a good idea to 
do that for ABCL, too, so I just put it in java.scm for now. Feel 
free to move it to its own package (or ask me to do so) if you'd 
rather. It might even fit better in lisp.scm.

Carlo