New C++11 tuple_cat (aka, libstdc++/50159)

[Paolo Carlini]

Hi,

the below is the code contributed by Daniel for the new, C++11 
conforming, tuple_cat, which I integrated and tested on x86_64-linux. 
Looks indeed very good to me.

If Daniel can still recognize it ;) - I took it directly from your first 
email - I mean to commit it later today and move to other work.

Then, on top of it, I would ask Daniel and / or Benjamin to (re-)do the 
constexpr bits, thus re-enable the corresponding test (get on tuple 
should be already ok for constexpr? I think this is not the case for 
pair and array, should be trivial to adjust)

Paolo.

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