8975: Use todo!() as placeholder body for generated match arms r=matklad a=jDomantas
`todo!()` seems to be a better fit for this than `{}`. Seeing that this assist predates stabilization of `todo` my guess is that simply no one bothered to change it yet.
Also fixed the issue where if the last arm was not block-like, rust-analyzer would not add a comma after it and would generate invalid code.
Co-authored-by: Domantas Jadenkus <djadenkus@gmail.com>