Show Unicode symbols for replaced control codes.

xs_json.h

@@ -260,9 +260,9 @@ static xs_val *_xs_json_loads_lexer(const char **json, js_type *t)
                     else
                         cp = i;
 
-                    /* replace dangerous control codes with the replacement char */
+                    /* replace dangerous control codes with their visual representations */
                     if (cp >= '\0' && cp < ' ' && !strchr("\r\n\t", cp))
-                        cp = 0xfffd;
+                        cp += 0x2400;
 
                     v = xs_utf8_enc(v, cp);
                     c = '\0';