Make (after keyword) indent-rules more flexible.

Make (after keyword) indent-rules more flexible.
* lisp/emacs-lisp/smie.el (smie-indent-offset-after)
(smie-indent-forward-token, smie-indent-backward-token): New functions.
(smie-indent-after-keyword): Use them.
(smie-indent-fixindent): Only applies to the indentation of the BOL.
(smie-indent-keyword): Tweak the black magic.
(smie-indent-comment-continue): Strip comment-continue before use.
(smie-indent-functions): Indent comments before keywords.