Extract exhaustiveness into its own crate
It now makes sense to extract exhaustiveness into its own crate! This was much-requested by rust-analyzer (they currently maintain by hand a copy of the algorithm), and I hope this can serve other projects e.g. clippy.
This is the churny PR: it exclusively moves code around. It's not yet useable outside of rustc but I wanted the churny parts to be out of the way.
r? `@compiler-errors`
dd0887c75c