Murisi/fix convert description parser rebase

[murisi]

Made miscellaneous adjustments in order to get namadac to work better with the hardware wallet. Specifically the following changes were made:

• Allow the creation of more convert randomness than there are convert descriptions. This might simplify the implementation of namadac where randomness is obtained separately prior to Transaction construction
• Fix the parser so that it parses the generators of convert description information objects. This issue was preventing the parsing of MASP Transaction builders.
• Now check whether the TxId digest computation throws any errors and propagates them. A safety measure to prevent computations from continuing with the wrong TxId digest.
• Fix the hashing of spend descriptions so that the anchor comes from the Sapling bundle. Note that its presence is guaranteed whenever there's at least one spend description.
• Weakened the conditions under which the Sapling bundle is hashed to just be whenever a spend, convert, or output description is present. This fixes the handling of Transactions with zero output descriptions.