Enrich StreamReader and SinkWriter

[nicflower]

Implementing feature suggested in issue #5745

Basically what was done is:

• implement Sink for StreamReader
• implement Stream for SinkWriterStream>
• implement AsyncRead for SinkWriter
• implement Stream for CopyToBytes

The implementation is pretty straightforward since the actual work is all demanded to the underlying object, so I thought that introducing new tests would not be really helpful.

[Darksonn]

We don't want Unpin bounds if they're not necessary. Also, the T parameter can be dropped.

Suggested change

	impl<S, T> Stream for CopyToBytes<S>
	where
	S: Stream<Item = T> + std::marker::Unpin,
	{
	type Item = T;
	fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
	let ref_mut = self.get_mut().get_mut();
	Pin::new(ref_mut).poll_next(cx)
	}
	}
	impl<S: Stream> Stream for CopyToBytes<S> {
	type Item = S::Item;
	fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
	self.project().inner.poll_next(cx)
	}
	}

[nicflower]

fixed

[Darksonn]

Please also fix it for SinkWriter and StreamReader.

[nicflower]

done

[Darksonn]

Thanks, looks good to me!