feat: add a snapshot field to model.State for original storage

[ClementWalter]

Why

Some functions of the state.cairo require an access to the previous state. This is done with the definition of a _snapshots entry in the EELS state.

What

Add a _snapshots: State* to the state definition and use this state in fetch_original_storage

How

    struct State {
        accounts_start: DictAccess*,
        accounts: DictAccess*,
        events_len: felt,
        events: Event*,
        transfers_len: felt,
        transfers: Transfer*,
        _snapshots: State*,
    }

• empty snapshot is just cast(0, State*)
• add a get_storage_original in state.cairo similar to read_storage but applied to the snapshotted state
• remove Account.fetch_original_storage and use this new method in sstore