✨ Add CurrencyAccount to handle all Currency related logic

[greymistcube]

The implementation is pretty convoluted at the moment. 🙄
There are numerous issues regarding Currency:

• It isn't at all clear who should be responsible of keeping a state "consistent" regarding a Currency.
  • "Consistency" is purely enforced by code, not stored data.
  • The whole Currency manipulation pipeline is running on top of the assumption that a state is "consistent"
  • There are two relations that can be checked purely from state + Currency:
    • Whether TotalSupply does not exceed MaximumSupply
    • Whether TotalSupply is being tracked and is correct.
• TotalSupplyTrackable loses its meaning starting with BlockMetadata.CurrencyAccountProtocolVersion.
• Read operation throwing an Exception for TotalSupply.
  • It is put in as a safety measure, but in reality, this does not achieve anything.
  • Handling of said Exception regarding TotalSupply must be handled from IAction side (or any outside service), and its interpretation requires additional knowledge about Currency and the behavior of read operation of TotalSupply.
  • Besides, it has never been used.
• Mint/burn authority validation has nothing to do with the underlying state.
  • One solution would be to enforce business logic through code analysis where a call to MintAsset() or BurnAsset() should be preceded by authority validation.
  • Falling short of that, the responsibility falls to the developers of IActions.

This is partially due to information about Currency not being recorded in a state and migration to CurrencyAccount model
does not permit adding in this missing information.

I'll be making some Currency spec and API changes in separate PRs to mitigate some of these issues. 😶

[greymistcube]

@OnedgeLee

Do this PR contains state migration test? Or, existing test can cover currency migration?

Migration is mostly covered by ActionEvaluatorTest.Migration.cs. Although now that I look at it, I should add Currency manipulation to the action used in MigrationThroughBlock test. 😗

[greymistcube]

@OnedgeLee

I've added an additional test regarding migration. See 4e5fc2f.