Support emplace with multiple arguments

[Askar Safin]

Hi. It seems that you wanted your "build" function of "variant.hh" to be named 
idiomatically, and so created new "emplace" function. Well, okey, but if you 
really want to be idiomatic, please, make it support multiple arguments. I. e. 
it should look so:

    template <typename T, typename... U>
    T&
    emplace (U&&... u)
    {
      return *new (yyas_<T> ()) T (std::forward <U>(u), ...);
    }

Of course, this will cover empty argument list case, too, so, if this 
definition is enabled, then you don't need "emplace ()".

==
Askar Safin
http://vk.com/safinaskar