spec: account requirements

[Mathieu]

Feature Request

During our incremental work on the account structure, we drafted the following requirements

• The account MUST be upgradeable, such that transactions are executed per the latest account class rules
• The account MUST get the latest class available from an external, upddateable registry
• The account update MUST be invisible to the user
• The account MUST be able to communicate with Cairo1 functions through an external Cairo1 Class
• The account MUST be able to call the Kakarot contract

We have some limitations at the protocol:

• The __validate__ function CANNOT make external calls
• As such, __validate__ will always be run in the context of the account's current class

From these rules, and limitations we can derive:

• The account WILL store the Cairo1Helpers class to be able to use it inside validate.
• The account WILL store the Kakarot address to be able to make calls to it
• The account WILL update its internal storage variables to the registry's latest values before executing a transaction
• The account WILL delegate the transaction execution to the latest class, if it is not the latest class
• The registry WILL have a frozen interface for the functions used in the accounts