Provide hooks to use a different tokenizer in SMIE.

Provide hooks to use a different tokenizer in SMIE.
* lisp/emacs-lisp/smie.el (smie-forward-token-function)
(smie-backward-token-function): New vars.
(smie-backward-sexp, smie-forward-sexp)
(smie-indent-hanging-p, smie-indent-calculate): Use them.
(smie-default-backward-token): Rename from smie-backward-token and
skip comments.
(smie-default-forward-token): Rename from smie-forward-token and
skip comments.
(smie-next-sexp): Handle nil results from next-token.
(smie-indent-calculate): Add a new case for special `fixindent' comments.