Fix issuance key derivation

[ConstanceBeguier]

Updated constants for master (extended) issuance key according to ZIP 227. Previously, we used the same personalization for the master extended spending key and the master extended issuance key, as well as the same purpose constant for the spending master key and the issuance master key.

Now, the following updates have been made:

• Personalization for the master extended issuance key: ZIP32ZSAIssue_V1
• Purpose constant for the issuance master key: 227"

[PaulLaux]

one change is required to make it perfect.

Also,

• please fix comment on line 190: /// Converts this issuance validating key to its serialized form
  to "spend key"

[PaulLaux]

I don't think we need this check.

[PaulLaux]

what we need is to make sure we can derive IssuanceValidatingKey, IssuanceAuthorizingKey potentially by calling

impl From<&IssuanceKey> for IssuanceAuthorizingKey {
    fn from(sk_iss: &IssuanceKey) -> Self {
        let isk = to_scalar(PrfExpand::ZsaIsk.expand(&sk_iss.0));
        // IssuanceAuthorizingKey cannot be constructed such that this assertion would fail.
        assert!(!bool::from(isk.is_zero()));
        IssuanceAuthorizingKey(conditionally_negate(isk))
    }
}

/// and...

IssuanceValidatingKey {

    /// Constructs an Orchard issuance validating key from uniformly-random bytes.
    ///
    /// Returns `None` if the bytes do not correspond to a valid key.
    pub fn from_bytes(bytes: &[u8]) -> Option<Self> {
        <[u8; 32]>::try_from(bytes)
            .ok()
            .and_then(check_structural_validity)
            .map(IssuanceValidatingKey)
    }
}

[ConstanceBeguier]

I updated the check for IssuanceAuthorizingKey.
I think deriving IssuanceValidatingKey from IssuanceAuthorizingKey never fails (=> no check required for this part)

[PaulLaux]

Approved with one comment.