Make TokenWithRefunds Production-Ready

[just-mitch]

Partial notes were used to make private refunds possible.

In short, private refunds allow Alice to fund an arbitrary transaction (possibly with public functions) using an arbitrary private token via a fee payment contract, and get her refund as private token notes.

Without this flow, Alice must fund her transactions either:

• natively with AZT herself
• using public balances and an FPC
• using private balances but forgoing a refund

Partial notes generally support patterns where we need to construct a note that is dependent on on-chain data sources that are not available in private.

Example sources are:

• price feeds
• prevailing interest rates
• weather data from IoT sensors
• time

Applications could include:

• start a point $(\text{owner} + \text{principle})*G$, complete it with the prevailing interest rate to construct a note representing a loan.
• submit a claim on your farm insurance based on regional flooding data.
• private discounts/deals based on time of day
• lock some private assets as a point, cash them out privately with interest based on the amount of time they were locked up.
• a dex where the liquidity is in public but rewards/swaps occur with notes

So the use cases are compelling, but there are problems with the current implementation. This issue tracks work to make the concept ready for production.

Tasks

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1. determine if using the x-coord of a Point for a note hash is safe #7325
   +0
2. bug(PartialNotes): Add verifiable link between to address/spending keys and the PrivateTokenNotes #7323
   A-security T-bug +2
3. bug(PartialNotes): use separate generators for the constituent pieces of PrivateToken #7320
   A-security T-bug +2
   
4. leak(PartialNotes): use different randomness for sponsored user and fee recipient in PrivateToken #7321
   A-privacy-leakage T-bug +2
5. bug(PartialNotes): Check funded_amount >= transaction_fee #7322
   A-security T-bug +2
   
6. bug(PartialNotes): account for key rotation in PrivateToken #7324
   A-security T-bug +2
7. unconstrained getter for SharedMutable #7326
   +0
   
8. Protect FPC against griefing #7650
   +0
   
9. Set flat fee to a reasonable value #7688
   +0

[benesjan]

Closing this now as #7650 is not planned to be tackled in the near future and everything else in this issue done.