Rust 1.18 release status

As of now there's one more beta patch in the queue, and that looks like it'll be the last:

Here are the remaining known beta regressions:

This one won't be resolved for beta but is minor:

There are two mingw-related regressions that came out of the LLVM upgrade and won't be reverted.

There's a PR to bump the prerelease version and get out a new beta. Then we'll run that through cargobomb.

Seems like we're doing ok. Thanks all.