Commits
Yosuke Shiro authored and Kouhei Sutou committed 3692a232166
ARROW-3858: [GLib] Use {class_name}_get_instance_private
Because `G_TYPE_INSTANCE_GET_PRIVATE` has been deprecated since version 2.58.
https://developer.gnome.org/gobject/stable/gobject-Type-Information.html#G-TYPE-INSTANCE-GET-PRIVATE
Author: Yosuke Shiro <yosuke.shiro615@gmail.com>
Closes #3017 from shiro615/glib-use-get-instance-private and squashes the following commits:
d5eedd91 <Yosuke Shiro> Use _get_instance_private()