[Patch] Implement std::experimental::variant

[Tim Shen]

On Mon, May 16, 2016 at 6:02 AM, Jonathan Wakely wrote:
>> Currently it's in std::experimental::fundamentals_v1. Is there a
>> better place to put it in?
>
>
> The p0088r2 proposal says namespace std, and that's what LEWG advised
> in Jacksonville. Let's add it to namespace std on trunk. We have
> plenty of time to move it to the experimental namespace (although it
> would not be _v1) during GCC 7 stage 1 if the plan changes.

I just realized that <type_traits> doesn't define aliases like
is_same_v, is_default_constructible_v, etc. Are they going to be in
C++17? Should I add them to <type_traits> as well?