(set_point): Considerable rewrite. Handle intervals both
before and after the old and new point values.
Redo handling of invisible intervals, and of motion hooks.
(textget): New function.
(graft_intervals_into_buffer):
create_root_interval needs Lisp object arg.
Set tree to new root interval.
Don't test TREE_LENGTH if buffer has no intervals.
Rearrange code to copy properties so that it really
does merge the inserted ones into the inherited ones.
(traverse_intervals): Pass `arg' on recursive calls.
(split_interval_left): Use new_length as basis for length of new.
(traverse_intervals): New arg ARG.
