Publish version 1.0

Crossbeam hasn't changed much in a long time. It may be time to publish 1.0 :)

My opinion is that all crates except for crossbeam-epoch have "peaked" and reached maturity. The thing about crossbeam-epoch is that its API is still not perfect and needs some improvement, mainly around custom collectors. But, even considering that, the API hasn't changed in a very long time, so why not just go for 1.0? If we decide to actually change it in the future, we can bump to 2.0.

I proposed we do the following now:

• Bump minimum required Rust version to 1.36.0 (the one with MaybeUninit).
• Fix CI.
• Decide what to do with #416
• Merge #431
• Delete deprecated Backoff::is_complete().
• Bump all crate versions to 1.0 and publish.

What does everyone think?