use Zeroize for keys/secrets

[redshiftzero]

Currently we use Zeroize only for the secret types in decaf377-ka (for no specific reason): https://github.com/penumbra-zone/penumbra/blob/main/crates/crypto/decaf377-ka/src/lib.rs#L17

It would be good to do a pass through the entire codebase and use Zeroize for keys and secrets.