Updated patch for std::list: Performance and memory usage improvements.

[Benjamin Kosnik]

This patch is in. I'm hoping that another one, like it for maps and sets
is forthcoming.

>Please, could you  name the functions/classes/data you're touching in
>your patches?  The above entry isn't really helpful :-(
>
>Do you have any measurements regarding your improvements?

I'd suggested this as well. I'm hoping something like this can be used
for the list, map, and set work:

http://gcc.gnu.org/ml/libstdc++/2003-03/msg00000.html

best,
benjamin