Add a lint against never type fallback affecting unsafe code
~~I'm not very happy with the code quality... `VecGraph` not allowing you to get predecessors is very annoying. This should work though, so there is that.~~ (ended up updating `VecGraph` to support getting predecessors)
~~First few commits are from https://github.com/rust-lang/rust/pull/123934https://github.com/rust-lang/rust/pull/123980~~