zeroize: possible UB in `Zeroize` implementation for `Option<T>`

Originally filed by @jessa0 as https://github.com/iqlusioninc/crates/issues/782:

It seems the Zeroize implementation for Option<T> where T: Zeroize has language-level UB here:

https://github.com/RustCrypto/utils/blob/00b569b52b6dece2a8ae881f8e5a150a45ccc674/zeroize/src/lib.rs#L330

I believe, as a repr(Rust) enum, the memory layout and set of valid bit-patterns for Option is not defined, and that setting an enum's storage to an invalid bit-pattern while a reference to it exists, even if the value is never read, is instant language-level UB. The documentation for Option does mention guarantees for several special cases, but the None case still isn't defined for many of those cases, and the Zeroize implementation is more generic than that. Here's an example of a miri error in such a situation, that scottmcm came up with on URLO here.

zeroize/src/lib.rs の330行目付近から始め、リンクされた Rust Playground または Miri の例を使って報告された動作を再現します。Option の実装に対する健全なアプローチを決定し、結果として得られる動作が報告された未定義動作エラーをもはや発生させないことを検証します。