Start work on rustix 1.0

[morr0ne]

Rustix has been on version 0.38.x for a while and work towards a 1.0 release seems to be a bit stale. There is already a 1.0 issue here #753 but it does not seem like any work has actually started. I feel like rustix should begin moving forward and start working on a 1.0 release.

Ideally the following things would happen

• The current main branch would start accepting breaking changes
• Create a new 0.38 branch based on the current main to keep supporting that version
• Start releasing "release candidates" for the 1.0 version on crates.io

The work required for a 1.0 release is quite lengthy has highlighted in #753, this would allow to start working on some of it.

I am aware the a 1.0-staging branch already exists but it currently only houses 2 commits which could be easily pulled into the main branch

Finally it would probably help adding a corresponding project and issue label to better track the changes