Zeroize sensitive CHURP data

[peternose]

Ensure that all CHURP structures containing sensitive data (e.g., key shares, derived keys, switch points, etc.) are zeroized on drop to prevent an attacker from retrieving any residual information.

[peternose]

It looks like p386 crate doesn't zeroize all sensitive data.

The crate uses crypto-bigint crate which constructs stack-allocated big integer types Uints using an array of Limbs or wrapped Words . The former can be zeroized, but the latter not. Because of that, some intermediate values in calculations are not zeroized. For example, function adc creates variables a and b of type WideWord, which are never zeroized.