Upgrade to a stable version of iovec

[dtolnay]

The iovec crate is critical to the public API of mio. It is how we coordinate vector reads/writes. As such, we cannot call mio stable until it is using a stable iovec.

[tamird]

This probably shouldn't have [help wanted]. iovec is also owned by @carllerche and looks to have significant unreleased changes. It's not clear what external contributors can do to help, here.

[carllerche]

@tamird this is a good point. There are no immediate plans to stabilize iovec. I'm going to close this issue.