armload of DEFINE-HASH-CACHE changes

armload of DEFINE-HASH-CACHE changes

 * To clear a cache, drop the entire vector instead of filling it with
   NILs: thread safe, less work, and doesn't add dirty pages to old
   generations.

   Entering a value after the cache has been dropped allocates a new
   one.

   Caches are now initialized with 0 instead of NIL -- faster to
   allocate.

 * Use DEFGLOBAL instead of DEFVAR.

 * SAVE-LISP-AND-DIE drops all caches.

 * UNSAFE-CLEAR-ROOTS drops caches depending on the depth of the
   collection: nursery collection keeps all caches, gen 1 collection
   drops the CTYPE-OF cache, gen 2 and deeper collections drop all
   caches.