refactor: extract fn get_mv_memory

[kien-rise]

One step towards https://github.com/risechain/pevm/issues/60

On the optimism branch, we will have:

    let mv_memory = 'b: {
        #[cfg(feature = "optimism")]
        if chain.is_optimism() {
            break 'b DeferDrop::new(get_optimism_mv_memory(&hasher, &block_env, &txs));
        }
        DeferDrop::new(get_mv_memory(&hasher, &block_env, &txs))
    };