Murisi/restructure tx

[murisi]

This is an attempt to achieve #1255 . Overall the main changes introduced by this branch are:

• Instead of having an outer transaction structure containing an inner transaction, there is now a single transaction structure containing a flat list of sections.
• Instead of wrapping inner transactions in a structure that supplies a signature, the unsigned data is added as a section to the transaction at the same level as it corresponding signature section.
• The WASM code now receives a Tx object instead of a data byte vector. This is because the data section's signature is stored in a different section, and because the extra data also resides in a separate section.
• Encryption is now done separately after all signing is complete - in particular this means that there are no longer any signatures over encrypted data.
• Upgraded the MASP client and validation code to use the latest MASP crate.
• Added a builder section to Txs where the auxiliary inputs that were used to construct a MASP transaction can be stored. This may be useful for hardware wallets when validating a given Transaction object.