Make intrusive_list_node's next/prev private

Tromey noticed that intrusive_list_node leaves its data members
public, which seems sub-optimal.

This commit makes intrusive_list_node's data fields private.
intrusive_list_iterator, intrusive_list_reverse_iterator, and
intrusive_list do need to access the fields, so they are made friends.

Change-Id: Ia8b306b40344cc218d423c8dfb8355207a612ac5
1 file changed