last stab at alternative pointer support

[Paolo Carlini]

... looks *much* better, thanks, I like the consistent dispatching via _Rb_tree_impl_chooser. Now we (libstdc++ maintainers) have to spend some time going through each single line again...

Two remarks: eventually everything would have to be uglified, thus not using_const_cast but something like __using_const_cast. Much more important: do you have a definite plan for containers like <vector>? I suspect many users expect the relatively simple <vector> to work well with alternate pointers and I hope there are no specific difficulties there!

Paolo.