A few more naming-convention fixes for getters and setters.

A few more naming-convention fixes for getters and setters.

* buffer.c (set_buffer_overlays_before): Move here from buffer.h,
and rename from buffer_overlays_set_before.
(set_buffer_overlays_after): Move here from buffer.h, and rename
from buffer_overlays_set_after.
* buffer.h (buffer_intervals): Rename from buffer_get_intervals.
All uses changed.
(set_buffer_intervals): Rename from buffer_set_intervals.
* intervals.c (set_interval_object): Move here from intervals.h,
and rename from interval_set_object.
(set_interval_left): Move here from intervals.h, and rename from
interval_set_left.
(set_interval_right): Move here from intervals.h, and rename from
interval_set_right.
(copy_interval_parent): Move here from intervals.h, and rename from
interval_copy_parent.
* intervals.h (set_interval_parent): Rename from interval_set_parent.
(set_interval_plist): Rename from interval_set_plist.
Return void, not Lisp_Object, since no caller uses the result.
* lisp.h (string_intervals): Rename from string_get_intervals.
(set_string_intervals): Rename from string_set_intervals.