refactor: keccak hashing & fixture adjustments

[alexbarnsley]

Summary

https://app.clickup.com/t/86dw1cxnq
https://app.clickup.com/t/86dw15f4f

• Updates Fixtures for transactions & identities
• Removes unused and undocumented crypto.transactions.Transaction class
• Updates hashing from sha256 to keccak256
• Fixes issues with trimming "0x" from the left of data - potentially causes issues whereby "0x0123" would become "123"
• Handles deserializing transactions outside of the consensus contract
• General tidying up & test updates

TODO:

• document decode_payload - https://github.com/ArdentHQ/docs-poc-protocol/pull/99

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged