We switch the `issueAuthSig` scheme from RedPallas without
key re-randomization to the Bitcoin Schnorr signature (as described in
bip340).
We also perform notation changes of `idk` to `imk`, and adjust the
derivation of the issuance keys to fit with the updated Issuance
Authorization Signature scheme.
This PR improves the naming of variables to make them consistent with
the broader ZIPs conventions.
It also makes changes to the issuance key derivation and the terms used
for the various keys.
Vivek Arte