Add unwrap shared_future constructor

[viboes]

Following N4123, add unwrap shared_future constructor

See also ​https://github.com/cplusplus/concurrency_ts/issues/58

shared_future(future<shared_future<R>>&& rhs) noexcept;

Effects:
    Constructs a shared_future object from the shared state referred to by rhs. The shared_future becomes ready when one of the following occurs:

        Both the outer future and the inner shared_future are ready. The shared_future inherits the value or the exception from the inner shared_future.
        The outer future is ready but the inner shared_future is invalid. The shared_future gets an exception of type std::future_error, with an error code of std::future_errc::broken_promise.

Postconditions:

        valid() returns the same value as rhs.valid() prior to the constructor invocation.
        valid() == true.
        rhs.valid() == false.