[v3] 25_algo merge/flatten from libstdcxx_so_7

[Paolo Carlini]

Hi all,

>Actually.... this moves over the rvalrefs/moveable testsuite additions
>from libstdcxx_so_7-branch to mainline. Chris, it would be great if you
>could look this over, but I'll assume it's ok if I've not heard
>anything by Thursday.
>  
>
Actually, seems a very good idea to me.

>This makes all moveable.cc tests dg-require-rvalrefs, which is a bit of
>a hokey dependency test for the library components, ie move. This seems
>simplest.
>
>Some slight changes to paths, __gnu_cxx::__make_move_iterator to
>std::make_move_iterator (and #include <iterator> if necessary),
>__gnu_cxx::__move to std::move (and #include <utility> if necessary),
>etc. 
>  
>
I can implement std::move_iterator and std::make_move_iterator in 
<iterator>, completing my previous (bit of) work on std::move &co in 
<utility>...

Paolo.