* sm-c-mode.el: Auto-align backslashes. Improve indent rules

* sm-c-mode.el: Auto-align backslashes.  Improve indent rules

* sm-c-mode/sm-c-mode.el: Auto-align backslashes.  Improve indent rules.
(sm-c--cpp-indent-comment-inside): New function.
(sm-c--cpp-smie-indent-functions): Use it.
(sm-c--cpp-smie-indent): Handle indentation within comments-within-cpp.
(sm-c-smie-grammar): Manually tweak precedence of ":label".
(sm-c-smie-operator-regexp): Parentheses aren't operators.
(sm-c--boi): Add `inner' arg.
(sm-c--boe): Complete the code.
(sm-c-smie--*-token): Refine.
(sm-c-smie-rules): Various tweaks based on tests on src/eval.c and
src/xdisp.c.
(sm-c--bs-changed): New var.
(sm-c--bs-after-change, sm-c--bs-realign, sm-c--bs-realign-1):
New functions.
(sm-c-mode): Use them.

* sm-c-mode/sm-c-mode-test.c: Add a few more cases.