Small zeroization fix

[emil-wire]

Just noticed that fn expand_keyin x-wing doesn't fully zeroize seed and sk_x_raw so I added explicit zeroization as a defense in depth measure.

[newpavlov]

Personally, I don't think we should explicitly zeroize temporary variables stored on stack since there is plenty of ways to spill sensitive data to stack and zeroize-on-drop does not help with moves (especially considering inefficient zeroization codegen for byte arrays, see: RustCrypto/utils#1504). Instead users should zeroize stack after sensitive computation is completed.

[emil-wire]

That makes sense. I'd be happy to close this PR if that's your stated policy. I just noticed other crates in this workspace already explicitly zeroize transient stack buffers after use, for example hqc-kem and dhkem, so this just follows that existing pattern.

Rezeroize_stack: it seems broader but also harder to apply idiomatically inside this crate - it requires estimating stack usage, using an #[inline(never)] wrapper, and is still documented as best-effort/codegen-dependent. I’m not sure expand_key is the right place to introduce an arbitrary stack scrub size.

[newpavlov]

I will leave this decision to @tarcieri.

I’m not sure expand_key is the right place to introduce an arbitrary stack scrub size.

Yes, this is why I wrote that stack zeroization should be done by users.