It's not an end-user tool

1) indexed_list no longer has virtual methods. It's not actually subclassed
and there is very rarely good reason to subclass collection classes.
2) Added a virtual dtor to indexed_list_object which is intended to be
subclassed. This allows derived dtors to be called if the object is
deleted with a indexed_list_object*.

* Disabled copying.
* Added const where appropiate.

This reverts commit 53b181dd.