ZIPs 226, 227, and 230: ZSA Transfer, Issuance and Burn

[vivek-arte]

This PR continues the discussion on the ZSA ZIPs from the previous #960 , all but the last commit of which was merged in #974.

It contains any recent changes to the following ZIPs:

ZIP 226: Transfer and Burn of Zcash Shielded Assets
ZIP 227: Issuance of Zcash Shielded Assets
ZIP 230: Version 6 Transaction Format

Note that the target issue for the ZSA ZIPs is #618.

The changes include:

• Movement of the burn information from outside the Action Group to inside the Action Group
• Renaming of some sub-branches of the txid_digest and signature_digest and auth_digest trees for clearer OrchardZSA separation.

[vivek-arte]

One of the changes in this PR shifts the information pertaining to the burning of assets into the OrchardZSA Action Group -- this occurs both in the Transaction Format and the computation of the TxID digest. A pictorial summary of this is included in this comment:

The above image shows, on the left, the structure of the OrchardZSA bundle before this PR, and on the right, the changes made to include the burn information into the Action Groups. The image below shows the changes that are to be made correspondingly to the digests:

[vivek-arte]

I also just pushed a commit which makes various improvements to the naming of the sub-branches of the TxID and Signature Digests, and the Authorizing Data Commitment. The pictures below give a quick overview of these changes.

[str4d]

FYI we pulled in the renaming commit into #987 because of the changes it implicitly made to where the sighash parts suggested by qedit were located, but we have reverted the renaming therein because it does not make sense once moved into the context of ZIP 246.