added tools to upgrade xsc001 tokens + approvals to new standard

[duelingbenjos]

Description

Tools to migrate XSC001 contracts (excluding currency) to the new approvals standard.

Previously approvals were stored in the balances Hash in the token contract, now they will be stored in a dedicated Hash, approvals.

• All methods (approve, transfer_from) which used the old way will be replaced with a new method that that uses the new approvals Hash
• All legacy approvals of the structure <contract_name>.balances:<account_1>:<account_2> were migrated to `<contract_name>.approvals:<account_1>:<account_2>
• Any tokens missing the balance_of method had it added.

Type of change

Please delete options that are not relevant.

•  Enhancement (simplifying, beautifying, better performance, etc.)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (change that requires code adjustments in other places)
•  Fork needed (fix or feature that requires a resync of blockchain state)

Checklist

• I have performed a self-review of my own code
• I have tested this change in my development environment.
• I have added tests to prove that this change works
• All existing tests pass after this change
• I have added / updated documentation related to this change