[Patch] Remove workaround for copy_backward

[Paolo Carlini]

Gabriel Dos Reis wrote:

>Paolo,
>
>  First, thanks for raising this issue.
>
>In the abstract, I think your patch is correct.
>
>However, in practice, I'll do differently.
>For compile-time efficiency reasons, I would not go through
>overloading resolution based on the last argument and function
>template partial ordering.
>
[snip]

I see... As you did (and wrote!) in cpp_type_traits.h, right?

I will work out the approach you suggest (thanks!) for both copy and 
copy_backward, right?

Anyway, if this is the way to go (by the way, are you deadly sure that 
our beloved compiler ;) is able to generate code equally efficient in 
the two cases?) I think we have to rethink some code in basic_string too...

Paolo.