Securely erase `k` in Schnorr.

[codygunton]

We do not ensure that the random nonce k is securely erased.

[kashbrti]

@ledwards2225 are we talking about erasing the ephemeral secret from memory? In that case I guess we're talking about the native code in crypto crate?

[ledwards2225]

Will be closed by AztecProtocol/aztec-packages#21106