Refactor Tx Module and support sapling

[prestwich]

• [X] Sapling transaction parsing
• [x] Sapling tx convenience functions (e.g. sighash)
• [X] Review spec for other changes (script, addresses, etc.)
• [X] Shielded Input Tests
• [X] Shielded Output Tests
• [x] Zkproof Tests
• [x] Joinsplit Tests
• [x] Sighash tests
• [x] So many tests

[prestwich]

@rrybarczyk this is ready for review. Any further changes will be tests

[prestwich]

Closes #69