(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.
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