Function checksum 

pub const fn checksum(frame: &[u8], crc_extra: u8) -> u16

Computes the checksum a frame carries: the CRC over its bytes with the message’s CRC_EXTRA folded in last.

Arguments

• frame - the frame bytes the checksum covers: everything after the start marker up to but not including the two checksum bytes (and not the signature).
• crc_extra - the CRC_EXTRA seed for the frame’s message id.

Returns

The 16-bit checksum to append, low byte first.