Massive Spend Refactor

[grumbach]

BREAKING CHANGE

Continuing off @maqi's redesign of Spends in refactor PR #1930 Redesigns the whole transacting flow from CashNotes all the way to Spend with a simpler design:

Removing:

• OfflineTransfer: all you need to make a transaction, signed
• TransferInputs : amassed data to make a transaction's inputs
• CashNotesAndSecretKey : this for inputs too
• TransactionBuilder: this makes a transaction from inputs
• CashNoteBuilder: this makes cashnotes from transactions
• UnsignedTransfer: this is a special case when we don't have a key to sign the transaction
• Transaction, Input, Output: this was in Spends
• the middle spend patch
• lots of unused error variants
• duplicate code
• layers and layers of obfuscation abstraction
• more than 500 lines of code
• ...

Replaced by:

• UnsignedTransaction: all you need to make a transaction, not signed
• SignedTransaction: all you need to make a transaction, signed

What's next

• [ ] adapt spend_simulation tests that were using the Transactions in Spends @RolandSherwin
• [x] implement UnsignedTransaction::verify
• [x] unit test UnsignedTransaction::new, UnsignedTransaction::sign
• [x] unit test hex serialization for both new types

[joshuef]

What's the impact here for OMNI disbursement if any?

[grumbach]

What's the impact here for OMNI disbursement if any?

This reworks the structure of the Spend and refactors the transaction flow, it shouldn't affect OMNI disbursement