refactor(token): optimize DataKey storage layout

[Richiey1]

This PR optimizes the DataKey storage layout in the token contract to reduce gas consumption and storage fees on Soroban.

Summary of changes:

• Assigned explicit, small integer discriminants to all DataKey enum variants. This ensures that the storage keys use the most compact representation possible.
• Cleaned up redundant variants and dead code in the lib.rs file to improve maintainability and further reduce the contract size.
• Verified logic consistency for allowances and balances.

Closes #70

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