Fix bug reported by Levente Meszaros sbcl-devel 2004-04-19:
cache filling with negative offset.
... when precomputing cache contents, don't include classes with
... irony of ironies: I'm pretty sure that there's a bug lurking
somewhere else in any case, because the problem was
being exhibited in the computation of a cache for
SLOT-BOUNDP-USING-CLASS, which doesn't (or shouldn't)
use a cacheing dfun, but instead has its own specialized
dfun which basically calls the boundp function from the
slot definition. Hmm...
... comments and minor tidying in cache.lisp