Split smie-indent-calculate into more manageable chunks.

Split smie-indent-calculate into more manageable chunks.
* lisp/emacs-lisp/smie.el (smie-indent-virtual, smie-indent-fixindent)
(smie-indent-comment, smie-indent-after-keyword, smie-indent-keyword)
(smie-indent-close, smie-indent-comment-continue, smie-indent-bob)
(smie-indent-exps): Extract from smie-indent-calculate.
(smie-indent-functions): New var.
(smie-indent-functions): Use them.