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regarding the "parent" handle.  These just separate out the different
usages based on the type of parent (interval vs lisp object); later
changes will do type checking and enforcement.

	* intervals.h (NULL_INTERVAL): Cast to INTERVAL type.
	(INT_LISPLIKE): New macro.
	(NULL_INTERVAL_P): Use it.
	(INTERVAL_HAS_PARENT, INTERVAL_HAS_OBJECT, SET_INTERVAL_PARENT,
	SET_INTERVAL_OBJECT, INTERVAL_PARENT, COPY_INTERVAL_PARENT,
	GET_INTERVAL_OBJECT, INTERVAL_PARENT_OR_NULL): New macros.

	* alloc.c (make_interval, gc_sweep): Use new macros; eliminate all
	explicit references to "parent" field of struct interval and
	associated unclean type conversions.
	* intervals.c (create_root_interval, root_interval, rotate_right,
	rotate_left, balance_possible_root_interval, split_interval_right,
	split_interval_left, interval_start_pos, find_interval,
	next_interval, previous_interval, update_interval,
	adjust_intervals_for_insertion, delete_node, delete_interval,
	adjust_intervals_for_deletion, merge_interval_right,
	merge_interval_left, reproduce_tree, graft_intervals_into_buffer,
	copy_intervals_to_string): Likewise.
	* intervals.h (AM_LEFT_CHILD, AM_RIGHT_CHILD, RESET_INTERVAL):
	Likewise.
	* syntax.c (update_syntax_table): Likewise.

	* intervals.c (reproduce_tree_obj): New function, like
	reproduce_tree but takes a Lisp_Object for the parent.  Declare
	with prototype.
	(graft_intervals_into_buffer): Use it when appropriate.
	(reproduce_tree): Declare with prototype.
	(balance_possible_root_interval): Check that the parent is a lisp
	object before trying to examine its type.