Supporting non-standard pointer types in STL containers

[Paolo Carlini]

Hi,

> At this point, I've been able to get a pointer type written,
> which along with changes to the bits/std_tree code of version 4.0.0
> of  libstdc++...

a quick feedback: any possible change in this area will go in mainline only. Thus, in order to allow the maintainers to further investigate this work, please provide the changes to stl_tree.h in in the form of a diff vs current mainline.

Thanks,
Paolo.