Don’t allocate char-table’s extra slots in regexp-out-charset

* lisp/emacs-lisp/regexp-opt.el (regexp-opt-charset): Do not use
'case-table as charmap char-table’s property.  The function has nothing
to do with casing and in addition using 'case-table causes unnecessary
extra slots to be allocated which ‘regexp-opt-charset’ does not use.

lisp/emacs-lisp/regexp-opt.el

@@ -236,7 +236,7 @@ CHARS should be a list of characters."
   ;; The basic idea is to find character ranges.  Also we take care in the
   ;; position of character set meta characters in the character set regexp.
   ;;
-  (let* ((charmap (make-char-table 'case-table))
+  (let* ((charmap (make-char-table 'regexp-opt-charset))
 	 (start -1) (end -2)
 	 (charset "")
 	 (bracket "") (dash "") (caret ""))