Expose SendStream::poll_write and poll_finish methods

[jean-airoldie]

Closes #1782

[Ralith]

I'm open to re-exposing named futures. That would provide access to the full breadth of write variations, rather than just the &[u8] case. On the other hand, it's a very good point that the various AsyncWrite impls commit us to supporting this pattern anyway, so we might as well expose it directly.

[jean-airoldie]

Personally I prefer having access to the polling functions directly because it makes multi-stage futures much simpler to implement.

[Ralith]

Figured; been there myself. That can break down if you want multiple futures to concurrently manipulate the same value, but writes require &mut SendStream anyway, so that isn't a concern here.

[jean-airoldie]

Cool. I'll do another one for the read polling, if you don't mind.

[Ralith]

SGTM. Best to be consistent, after all.

[ibigbug]

is this removed again in the latest release? this one is relying on it https://github.com/hyperium/h3/blob/master/h3-quinn/src/lib.rs#L600

@djc @Ralith

[djc]

hyperium/h3#238 already ports h3-quinn to quinn 0.11.