[v3] Declare std::tuple_cat constexpr (+ all std::get)

[Paolo Carlini]

Hi,

Daniel kindly helped adjusting <tuple> to enable constexpr for 
std::tuple_cat. At the same time all the std::get overloads, for 
std::pair and std::array too.

Tested x86_64-linux, committed.

Thanks,
Paolo.

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