datalog: tokenizer raises on unexpected characters (256/256)

Bug: characters not recognised by any branch of `scan!` (`?`,
`!`, `#`, `@`, `&`, `|`, `\\`, `^`, etc.) were silently consumed
via `(else (advance! 1) (scan!))`. Programs with typos would
parse to a stripped version of themselves with no warning —
`?(X).` became `(X).` and produced confusing downstream errors.

Fix: the else branch now raises a clear "unexpected character"
error with the offending char and its position.

1 new tokenize test.

lib/datalog/tokenizer.sx

@@ -254,7 +254,9 @@
                       (dl-emit! "op" "/" start)
                       (advance! 1)
                       (scan!)))
-                  (else (do (advance! 1) (scan!)))))))))
+                  (else (error
+                          (str "Tokenizer: unexpected character '" ch
+                               "' at position " start)))))))))
       (scan!)
       (dl-emit! "eof" nil pos)
       tokens)))