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

[drips-wave]

@Richiey1 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[Richiey1]

Hi maintainers! 👋

I've pushed the latest fixes to address the dependency conflicts and test errors:

• Optimized the DataKey storage layout for gas reduction.
• Standardized the Cargo.toml dependencies to align with soroban-sdk v22.0.11 and resolved the e2e workspace conflicts.
• Fixed the syntax errors and unclosed delimiters in the token tests.

Could you please re-run the CI checks from your end and take a look at the changes? Let me know if there's anything else needed! Thanks.