(back_comment): Use one switch rather than a few `if's.

(back_comment): Use one switch rather than a few `if's.
Obey open_paren_in_column_0_is_defun_start.
When reverting to the `slow' method, try to nicely handle the case
of nested comments by checking that the comment-starter we found
does indeed match the comment-ender.
(scan_sexps_forward, scan_sexps_forward):
Ignore excessive opening parenthesis rather than throwing an error.