Changes towards better type safety regarding intervals, primarily

Changes towards better type safety regarding intervals, primarily
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.