Implement vectored I/O for TcpStream

[Thomasdezeeuw]

@carllerche I know you've said your not a fan of this approach but at least we can start using more nightly features on the crate without breaking the stable builds this way.

TODO:

• Remove TcpStream::{read_bufs, write_bufs}.
• Windows implementation, already uses IoVec so it shouldn't be too hard.
• Remove iovec dependency.

Updates #957.

[Thomasdezeeuw]

I want to take the same approach for #977.

[Thomasdezeeuw]

@carllerche What do you think about it?

[Thomasdezeeuw]

Well 1.36 was just released, so this doesn't have to work with nightly features any more. But this can still apply to peer_addr.

[carllerche]

@Thomasdezeeuw my take is, let’s target a future rust release and use nightly until then. I just prefer avoiding the feature flag. I expect we won’t cut 0.7 until at least async / await is on stable.

[carllerche]

Basically I’m fine with this change, just without the feature flag :-)

[Thomasdezeeuw]

@carllerche Sounds good to me. Could you change the CI to use the nightly compiler only? I haven't figured out the Azure pipeline stuff.

[Thomasdezeeuw]

I've forced push changes to just implemented {read, write}_vectored. But this will fail the CI on old stable compilers.

[carllerche]

We can probably just switch stable to nightly in Ci for now. That is what we do for Tokio.

[Thomasdezeeuw]

Looking at another build from earlier today (https://dev.azure.com/tokio-rs/Tokio/_build/results?buildId=1324&view=logs), it seems 1.36 is only used for macOS builds, other OSes are still on 1.35. I'll wait off on a rebase when the other OSes also use 1.36.

[carllerche]

@Thomasdezeeuw this comments explains how to get CI on the latest Rust.

[Thomasdezeeuw]

Pr #1028 fixes the problems with the CI, so I'll wait for that to be merged before rebasing this.

[Thomasdezeeuw]

I've rebased on master, @carllerche can you give this a final look over to see if I didn't mess anything up while merging?

[Thomasdezeeuw]

Great, both min Rust version and Windows fails because they both use 1.35.