Add SML-mode.

git-subtree-dir: packages/sml-mode
git-subtree-mainline: a50c62a632aa422957a0e4224d5c6e4c2366156b
git-subtree-split: d2ef91a138487b521bb7e08722b8d122aa757b5a
* sml-mode.el: Integrate BUGS&NEWS; re-add run-sml.
* packages/sml-mode/sml-mode.el (sml-mode-map): Add back C-c C-s binding.
(sml-prog-proc-switch-to): Add missing interactive spec.
* sml-mode.el (inferior-sml-mode): Fix comint-completion-addsuffix.
* sml-mode.el (sml-mode-variables): Set sml-prog-proc-descriptor here...
(sml-mode): ... instead of here.
* sml-mode: Release 6.3, bugfixes
* packages/sml-mode/sml-mode.el (sml-imenu-regexp): Make it a const.
(sml-imenu-create-index): Don't assume we'll find an = after structure.
* sml-mode.el (sml-smie-rules): Adjust for new :close-all rule.
* sml-mode/sml-mode.el (sml-smie-grammar): Add include&sharing.
(sml-smie-rules): Add functor rule.
(sml-smie-definitional-equal-p): Use smie-backward-sexp.
* sml-mode.el (sml--rightalign-and-p): New function.
* sml-mode/sml-mode.el: Release new version.
* packages/sml-mode/sml-mode.el (sml-smie-grammar): Add "withtype".
(sml-smie-rules): Use pcase.
(sml-smie-non-nested-of-p): Rewrite to avoid regexp and stay closer to point.