SBCL internals changed

[stassats]

SBCL internals related to structures have changed, breaking cl-store, in particular, the struct-class.1 test.

[lmj]

Since this causes a compilation error with the upcoming SBCL release, I submitted a "fix" which removes the feature altogether (#7). I've no prejudice against someone submitting a real fix, though I would hope the dependency on internals would not be as extreme as before.