Inline functions to examine and change buffer intervals.

Inline functions to examine and change buffer intervals.
* alloc.c (mark_interval_tree): Remove.
(MARK_INTERVAL_TREE): Simplify.
(UNMARK_BALANCE_INTERVALS): Remove.  Adjust users.
* intervals.c (buffer_balance_intervals): New function.
(graft_intervals_into_buffer): Adjust indentation.
(set_intervals_multibyte): Simplify.
* buffer.h (BUF_INTERVALS): Remove.
(buffer_get_intervals, buffer_set_intervals): New function.
* alloc.c, buffer.c, editfns.c, fileio.c, indent.c, insdel.c:
* intervals.c, textprop.c: Adjust users.