Methods to calculate and verify checksums

[tgross35]

Could methods for calculating and verifying checksums be added to packets? Something like:

impl MutableXPacket {
    /// Calculate the checksum and set the field
    fn update_checksum(&mut self);

    /// Return `true` if the checksum is valid for the current data, otherwise `false`
    fn verify_checksum(&self) -> bool;
}