Small changes to the deposit tx primitive

[clabby]

Overview

Small changes to the deposit transaction primitive:

• Deposit transactions don't have nonces; This value always defaults to 0.
• Add getters for source_hash, mint, & is_system_transaction that may be enabled by the optimism feature flag.
• Temporarily panic when encoding a deposit transaction with a non-zero signature. We should handle this higher up, panicking here isn't the right move.