[Patch] Implement is_[nothrow_]swappable (p0185r1)

[Daniel Krügler]

This is an implementation of the Standard is_swappable traits according to

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0185r1.html

During that work it has been found that std::array's member swap's exception
specification for zero-size arrays was incorrectly depending on the value_type
and that was fixed as well.

- Daniel
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