Transition away from arithmetic-2.

 - Switched all books that were still using arithmetic-2 to instead
   use arithmetic-3, except for:

     projects/concurrent-programs/bakery/stutter2.lisp

   because some proof broke when I tried it and I didn't try to
   debug it.  (It might be easy, or not.)

 - Stop building arithmetic-2 as part of the "basic" target since
   it isn't so important.

Just getting rid of arithmetic-2 altogether seems like a better plan
than to try to resolve its name clashes with, e.g., arithmetic-3.