Function checksum_skipping 

pub fn checksum_skipping(bytes: &[u8], skip: usize) -> u32

CRC32C over bytes, with the four bytes at skip treated as zero.

Every checksum in the format is defined this way, so it is one function rather than a convention each structure re-implements. The field is skipped rather than excluded so that the covered range stays contiguous and stays easy to state in the specification.