0.9.18.21:

0.9.18.21:
        Fix DEFUN-CACHED thread/interrupt-safety issues. It was
        possible for FOO-CACHE-ENTER to modify a region in the cache
        vector while FOO-CACHE-LOOKUP had only partially read it.

        * Instead of storing all the data in one vector, the main
          cache vector will only contain references to bucket vectors,
          which contain the real data. FOO-CACHE-ENTER will always
          allocate a new bucket, old buckets are never modified. Thus
          FOO-CACHE-LOOKUP consistently sees either a pre- or post-
          FOO-CACHE-ENTRY state.