[v3] constexpr array additions

[Benjamin Kosnik]

Also pointed out by Daniel. 

It's kind of hokey to work around -fno-exceptions like this, but seems
harmless. In this particular case, deferring the throw to a function
like __throw_out_of_range makes the ::at member an invalid constant
expression.

Yay! std::array with compile time operator []!

tested x86/linux

-benjamin
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