fn key_for_erc20_balance(address: &Address) -> H256 {
    let address_h256 = address_to_h256(address);

    // 20 bytes address first gets aligned to 32 bytes with index of `balanceOf` storage slot
    // of default ERC20 contract and to then to 64 bytes.
    let slot_index = H256::from_low_u64_be(51);
    let mut bytes = [0_u8; 64];
    bytes[..32].copy_from_slice(address_h256.as_bytes());
    bytes[32..].copy_from_slice(slot_index.as_bytes());
    H256(keccak256(&bytes))
}
Create a `key` part of `StorageKey` to access the balance from ERC20 contract balances
core/lib/types/src/utils.rs