Add emplace promise::set_value and emplace make_ready_future

[viboes]

I wonder if we shouldn't have a emplace promise::set_value (as std::experimental::optional<T> has) and a emplace make_ready_future.

template <class ...Args>
promise<T>::set_value(Args&& ... args);
 template <class T, class ...Args>
future<T> make_ready_future(Args&& ... args);