Implement transaction building and verification

[joebebel]

Our transactions are substantially more complex than Zcash transactions.

Tx building:

• [x] Proving all Transfers
• [x] Proving all VPBlinds
• [x] Proving all VPs
• [ ] Tx signatures
• [x] Combining multiple partial tx into one tx
• [ ] Support aggregating proofs

Tx verification:

• [x] Verifying all Transfers
• [x] Verifying all VPBlinds
• [x] Verifying all VPs
• [ ] Checking tx signatures
• [ ] Ensuring all VPs get called with correct public inputs (partial tx partitioning correct)
• [ ] Support ark-plonk circuits/proofs

Misc:

• [x] Document tx building in spec
• [x] Document tx verification in spec
• [x] Make tests for tx building
• [x] Make tests for tx verification