Add repr(C) to unions to ensure variants are laid out at the start

[5225225]

I found this bug when investigating use cases for the lint proposed rust-lang/rust-clippy#8289 (comment)

If a union doesn't have a #[repr(C)] attribute on it, the two variants do not have to be laid out both starting at offset 0. It would be perfectly valid to treat that union keyword as a struct keyword, and then suddenly you're reading uninit data / getting the wrong results.

Source for this: https://doc.rust-lang.org/reference/types/union.html says

The memory layout of a union is undefined by default, but the #[repr(...)] attribute can be used to fix a layout.

[kornelski]

Great catch! Thank you