Consider releasing 1.0 preview/release candidate versions

[morr0ne]

As rustix approaches its 1.0 release, I'd like to suggest publishing preview or release candidate versions to crates.io. This would really help smooth the transition and provide valuable feedback before the final release.

Publishing preview versions would bring several big benefits:

• Let users migrate their code gradually instead of dealing with a sudden 1.0 release
• Get early real-world testing and feedback while we can still easily make changes
• Enable projects (including several of mine) to publish proper releases instead of being stuck with git dependencies
• Give us room to refine APIs based on actual usage before committing to stability

For versioning, we could use standard semver pre-release tags like 1.0.0-alpha.1, 1.0.0-beta.1, or 1.0.0-rc.1 - this ensures cargo won't automatically update to newer versions without explicit intervention.

This is ofc just a suggestion, but I really think it would help the project a lot. The main reason I'm bringing this up is that I currently maintain/work on quite a few projects that depend on rustix, and it would make life much easier to pin to specific versions instead of relying on git dependencies

[sunfishcode]

I've now published rustix 1.0.0-prerelease.0. Please do test it out and report any findings!

I've documented all the API changes I'm aware of here; please let me know if anything there is unclear or if there's something not covered.

A potential change that hasn't landed yet is #1290. It's mostly a backwards-compatible change, however it is technically an API break because in some cases it requires users add a &mut *. If that change lands, I'll plan to do another prerelease before the final release.

[sunfishcode]

Please follow #753 for further updates.