std::get overloads?

Defining overloads in `namespace std` is undefined behavior, so I'm wondering why we're adding overloads of `std::get` for `boost::array`. Since we aren't specializing `std::tuple_size` or `std::tuple_element`, these overloads would never be used anyway, even if they were legal.

No file or test is named in the issue. Start by locating the boost::array std::get overloads and the std::tuple_size/std::tuple_element declarations, then inspect related tests and project conventions; done means the legality and usability concern has an explicit resolution.

索引モデルが issue の本文から書いたものです。