initial commit

Somewhat modified version of specializable.lisp from
http://www.lichteblau.com/git/?p=specializable.git
[ commit eb30d235951c3c1d128811278760f1db36cd336c ]

Lightly modified on a first read-through, mostly cosmetic though with
the innovation of the GENERALIZER concept, as in
COMPUTE-APPLICABLE-METHODS-USING-GENERALIZER and
SPECIALIZER-ACCEPTS-GENERALIZER-P.  Lots of FIXME notes for the things
that weren't trivial.