Using `tx_hash` for transaction signing

Summary

This minor release updates to Cairo 0.8.1 which now exposes the transaction hash as part of the tx_info object. Following that change, it updates the ArgentAccount contract to use the transaction hash in the signature verification.

What's Changed

• Use transaction hash for signing by @juniset in #34
• Fix imports for 0.8.1 by @juniset in #36
• Removed unused import on test by @CremaFR in #38

New Contributors

• @CremaFR made their first contribution in #38

Full Changelog: v0.2.0...v0.2.1