Use #[repr(C)] so the offset of every union field is 0.

[anforowicz]

According to
https://rust-lang.github.io/unsafe-code-guidelines/layout/unions.html#unions-with-default-layout-reprrust "the default layout of Rust unions is, in general, unspecified" and "there are no general guarantees about the offset of the fields, [and] whether all fields have the same offset".

This commit explicitly annotates union A with #[repr(C)] so that both of its fields are at offset 0 - this ensures that transmuting one field into the other is valid. This change is based on https://rust-lang.github.io/unsafe-code-guidelines/layout/unions.html#c-compatible-layout-repr-c which says that "the offset of every field is 0".

[anforowicz]

Credit for finding the lack of offset guarantees goes to @danakj - this PR is based on the comments at https://chromium-review.googlesource.com/c/chromium/src/+/4723145/31/third_party/rust/chromium_crates_io/vendor/crc32fast-1.4.2/src/specialized/pclmulqdq.rs#69

This initial PR proposal is a fairly minimal change to address the problem. I've also considered the following alternative approaches for fixing this issue:

• Do nothing:
  • The modified code is behind if false and therefore is not used
  • This issue would still be present if the code is uncommented + would still be brought up in future reviews/audits
• Switch to std::mem::transmute
  • Maybe this would be more readable?
  • https://doc.rust-lang.org/std/mem/fn.transmute.html says that compilation will fail if both types do not have the same size (this is not explicitly checked in the union approach)
• Delete the unused code altogether

WDYT?