Basic functions to set Lisp_Object and pointer slots of intervals.

Basic functions to set Lisp_Object and pointer slots of intervals.
* intervals.h (interval_set_parent, interval_set_object):
(interval_set_left, interval_set_right, interval_set_plist):
(interval_copy_parent): New function.
(SET_INTERVAL_OBJECT, SET_INTERVAL_PARENT, INTERVAL_PTR_SIZE): Remove.
(RESET_INTERVAL, COPY_INTERVAL_CACHE, MERGE_INTERVAL_CACHE): Adjust
indentation.
(INTERVAL_SIZE): Remove.  Adjust users.
* alloc.c, intervals.c, lread.c, textprop.c: Use new functions.