Added support for "toothbrush" applications, i.e., building ACL2 applications using a relatively small subset of the ACL2 sources.

Note that ACL2 source file basis.lisp has been replaced by
basis-a.lisp and basis-b.lisp, where basis-a.lisp is part of each
"toothbrush".  To that end, other code, not originally in basis.lisp,
has also been moved into basis-a.lisp.

See the new community books directory books/system/toothbrush/ for
toothbrush stuff.  In particular, subdirectory tests/ has some
examples.  I expect to add additional examples for des and aes, after
those contributions from Warren Hunt have been added (perhaps under
books/projects/security/).

Documentation will likely be forthcoming.  For now, see
books/system/toothbrush/README, which shows how to run the examples
and gives disclaimers.  The examples will be run as part of ccl and
sbcl regressions.