Patch for stl_tree.h and cpp_type_traits.h.

[Gabriel Dos Reis]

Dhruv Matani <dhruvbird@gmx.net> writes:

| This patch basically applies the same things to the rb_tree as the other
| container + one more thing which is to detect whether the comparison
| function is actually a function or an optimizable struct/class which can
| be derived from, and does accordingly. Also, the __is_pod struct has
| been changed. I have however (without telling anyone!) introduced a new
| namespace, which might not always be a welcome change. It's called
| __gnu_type_traits, because I could not find another way out! The

Anything that is private to our implementations is now being put in
__gnu_internal, IIRC. 

-- Gaby