(make_interval, Fmake_symbol, allocate_misc):

(make_interval, Fmake_symbol, allocate_misc):
Initialize the new field `gcmarkbit'.
(mark_interval, MARK_INTERVAL_TREE): Use the new `gcmarkbit' field.
(mark_interval_tree): Don't mark the tree separately from the nodes.
(UNMARK_BALANCE_INTERVALS): Don't unmark the tree.
(mark_maybe_object, mark_maybe_pointer, Fgarbage_collect, mark_object)
(survives_gc_p, gc_sweep): Use new `gcmarkbit' fields.