1.0.23.62: fix bug 357

1.0.23.62: fix bug 357

 Originally reported by Bruno Haible, more recently by Stephen Wilson.

 * SHARED-INITIALIZE (SLOT-OBJECT) should not check structure slots
   versus +SLOT-UNBOUND+: uninitialized slots are zeroed. Since adding
   slots to structure classes cannot cause those slots to be added to
   structure instances, we don't really have to check for boundness at
   all.

 * SB-PCL::STRUCTURE-TYPE-SLOT-DESCRIPTION-LIST and
   SB-PCL::MAKE-STRUCTURE-CLASS-DEFSTRUCT-FORM did not take overridden
   slot specifications into account, and the latter also omitted
   initform and type information.

 * Delete SB-PCL::ALLOCATE-STRUCTURE-INSTANCE, unused.

 * ALLOCATE-INSTANCE (STRUCTURE-OBJECT) should not fall back on
   ALLOCATE-STANDARD-INSTANCE.