fix: Serde impls for kyber

[OtaK]

Hi!

Here's a small fix for the Serde impls regarding Kyber key material

[bwesterb]

Thanks of this. What was the problem you hit?

[OtaK]

Basically enabling the serde_impls feature flag wouldn't work as serde doesn't have blanket impls for large, fixed size arrays (it is pending on const generics in traits), hence the additional dependency.

And if you're storing the keys anyhow (encrypted at rest storage and/or for instance, storage of an MLS ratchet tree) you usually want to have a serde implementation to be able to serialize the data. That's when I hit the issue - and fixed it.

[bwesterb]

Tested. Thanks!