(rx (| numeric alphabetic)) yields a char-set: but

(rx (| ,(rx numeric) ,(rx alphabetic))) doesn't:, even
though the "type-checking rules" for char-sets says
that it should:

> (rx (| numeric alphabetic))
'#{re-char-set}
> (rx (| ,(rx numeric) ,(rx alphabetic)))
'#{Re-choice}

The following patch fixes this but I'm not sure if
make-re-choice is the right place to tackle this problem:

This fixes bug report #1063781

scsh/rx/re.scm

@@ -133,6 +133,8 @@
   (really-make-re-choice elts tsm (new-cre posix-str tvec)))
 
 (define (make-re-choice res)
+  (if (every re-char-set? res)
+      (make-re-char-set (apply char-set-union (map re-char-set:cset res)))
       (make-re-choice/tsm res
                           (fold (lambda (re sm-count)
                                   (let ((maybe-tsm (re-tsm re)))
@@ -140,7 +142,7 @@
                                              (number? sm-count))
                                         (+ maybe-tsm sm-count)
                                         (unspecific))))
-			    0 res)))
+                                0 res))))
 
 ;;; Slightly smart choice constructor:
 ;;; - Flattens nested choices