tokio-rustls: bring back writev

[hawkw]

Since Tokio 0.3 removed support for vectored IO, tokio-rustls will no longer use writev when available (see #29). When tokio-rs/tokio#3135 is resolved, and Tokio re-adds support for vectored IO, it would be great to get writev support back in tokio-rustls.

In an ideal world, the solution with the new API in Tokio would no longer require specialization, so vectored IO could be supported on the stable compiler as well.

[hawkw]

I'd be happy to work on this, though it is obviously blocked on tokio-rs/tokio#3135.

[quininer]

@hawkw Hi, are you still interested in this?

[hawkw]

@quininer yup, just a bit busy with some other things. I'd like to take a look at this in the near future unless anyone beats me to it!