[v3] 25_algo merge/flatten from libstdcxx_so_7

[Paolo Carlini]

... one note however: if I add an appropriate std::move_iterator, the 
current configure test for rvalref will succeed and, in turn, the 
testcases counting the number of copies saved by moves will fail because 
the actual algorithms and containers have not been implemented. 
Therefore, incrementally, I plan to xfail those just-added tests 
actually counting copies. If people can figure out better incremental 
plans, I'm all ears...

Paolo.