Function compute_arbos_mixhash 

pub fn compute_arbos_mixhash(
    send_count: u64,
    l1_block_number: u64,
    arbos_version: u64,
) -> B256

Compute the mix hash from the three u64 components.

Layout: ]send_count (8 bytes)]]l1_block_number (8 bytes)]]arbos_version (8 bytes)]]0..0]