(chibi parse): Ensure reason is always a string

While testing a `(chibi parser)`-based parser I noticed that
`parse-string` is the only provided parser combinator which uses a list,
instead of a string, as a failure reason. It is not explicitly
documented but since all other standard parser combinator use a string
for the error reason I assume this to be a bug and have adjusted the
`parse-string` combinator accordingly in this commit.

lib/chibi/parse/parse.scm

@@ -663,7 +663,7 @@
 (define (parse-string str)
   (parse-map (parse-with-failure-reason
               (parse-seq-list (map parse-char (string->list str)))
-              `(expected ,str))
+              (string-append "expected '" str "'"))
              list->string))
 
 ;;> Parse a sequence of characters matching \var{x} as with