Fix LispWorks structure-object format incompatibility

    (ql:quickload :flexi-streams)
    (ql:quickload :cl-store)
    
    (defstruct foo x)
    
    (defparameter *foo* (make-foo :x 3))
    
    (defparameter *seq* (flexi-streams:with-output-to-sequence (out)
                          (cl-store:store *foo* out)))
    
    (defun seq->obj (seq)
      (flexi-streams:with-input-from-sequence (in seq)
        (cl-store:restore in)))
    
    (defun self-check ()
      (equalp *foo* (seq->obj *seq*)))

Manually copying LispWorks' value of `*seq*` and pasting it into `(seq->obj <pasted data>)` on SBCL or CLL or Allegro gives

    Number of slots restored does not match slots stored.
       [Condition of type SIMPLE-ERROR]

The reverse direction produces a nonsensical error on LispWorks,

    LOOP repeat counter X not a number.
       [Condition of type CONDITIONS:SIMPLE-PROGRAM-ERROR]

Tested on LW Personal Edition 6.0 and 6.1.