Implement multi-asset support

[joebebel]

• [x] Add asset_type field to Note
• [x] Add token_vp public input to the circuit with verification against asset_type
• [x] Add data field to Note (for NFTs, etc)
• [x] Ensure asset_type and `data are committed to in the note commitment
• [ ] Add asset_type and data to encrypted note output
• [ ] Remove or fix support for dummy notes in the multi-asset model
• [ ] Document in spec
• [ ] Make multi-asset tests

This now also depends on implementing example token VPs in #5 .