First "working" version of cacheing comments in a text property.

First "working" version of cacheing comments in a text property.

To test this facility, set `comment-cacheing-flag' to non-nil.

* src/buffer.c, src/buffer.h:  Create BVAR variable `comment-depth-hwm'.

* src/insdel.c (signal_after_change): call Ftrim_comment_cache.

* src/syntax.c (comment-depth-hwm): Removed (moved into buffer.c/h).
(scan_sexps_forward): Restore to the previous functionality by removing the
parameter `propertize' and the code which applied text properties.
(Ftrim_comment_cache, syntax_table_value_is_interesting_for_literals)
(check_comment_depth_hwm_for_prop, scan_literals_forward_to): New functions.
(back_comment): Extensive rework.  It now calls old_back_comment only when
`comment-cacheing-flag' is non-nil.
(comment-depth-values): New variable.

* src/syntax.h (check_comment_depth_hwm_for_prop): Exported.

* src/textprop.c (set_properties, add_properties, remove_properties): * call
check_comment_depth_hwm_for_prop.