Simpler Select implementation

[Lucretiel]

I'd like to propose a variant on select, perhaps called first or race (the latter copied from javascript). Unlike select, it would simply discard other results from incomplete futures. This allows it to have a much simpler implementation, and to allow non-unpin futures. It'd look like this:

#![derive(Debug, Clone, Default)]
struct First<A, B> {
    future1: A,
    future2: B
}

impl<A: Future, B: Future> Future for Select<A, B> {
    type Output = Either<A::Output, B::Output>;

    fn poll(self: Poll<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
        // This will obviously have to use a pin projection
        match self.future1.poll(cx) => {
            Poll::Ready(a) => Poll::Ready(Either::Left(a)),
            Poll::Pending => self.future2.map(|b| Either::Right(b)).poll(cx),
        }
    }
}

impl<A: FusedFuture, B: FusedFuture> FusedFuture for Select<A, B> {
    fn is_terminated(&self) -> bool {
        self.first.is_terminated() || self.second.is_terminated()
    }
}

The use case here would be for combining with more ephemeral futures, such as timeouts, or any other case where you only care about the first completed future, rather than trying to track and preserve the set of futures being selected over.

[Lucretiel]

Never mind, this is a duplicate of #2031