feat: access initial state in the middle of a transaction

[ClementWalter]

Why

In the kakarot implementation, we use Starknet contract storage as the backend for data and fetch data on the fly when required.

What

• In the os.cairo, create a initial_state = state before apply_transactions
• give this in implicit argument to apply_transactions and Interpreter.execute
• add to the Message struct the initial_state
• when finalizing a context, the parent message needs to be updated with the initial_state of the child
• in the end of the os, after apply_transactions, finalize the initial_state
• read from the initial_state in exec_sstore instead of Account.fetch_original_storage :

tempvar initial_state = evm.message.initial_state
State.read_storage{state=initial_state}(evm.message.address, key)
tempvar message = Message(..., initial_state=initial_state, ...)

• remove Account.fetch_original_storage
• for the test, add in test_memory_operations.cairo 2 cases for sstore: one with original_value == current_value and one with original_value != current_value