[wit-bindgen-rust] Fix 2024 edition compat

[yoshuawuyts]

I believe this should fix the failing CI in bytecodealliance/cargo-component#372, triggered by the new edition. I'm not entirely sure why the tests seem to be opting into a new edition, but I guess they do? Regardless: this changes the codegen of wit-bindgen-rust to respect the new "unsafe fn does not imply unsafe {}" rule added in the 2024 edition.

This PR is best reviewed commit by commit. The first commit is just the output of cargo fix --edition. The second commit are the actual unsafe changes. (edit: and the third commit reverts the 2024 edition style changes to make CI pass).

Thanks!

[alexcrichton]

Would you be up for adding a test for this? For example leaving the wit-bindgen crate itself as edition 2021 but changing the generated output to be compatible with 2024. I'd ideally like to change r#gen to generator since raw idents aren't the actual fix to have here (just the quick fix) down the road, and I'd also like to double-check that the generated output is indeed compatible with 2024 since IIRC we also need to do unsafe extern "C" { .. } I think?