Improve typing for CAN/CAN-FD

[pd0wm]

Try to make impossible states not representable, but hopefully not by making the UX worse. E.g.

• Remote frames cannot contain any data. Separate type?
• Remote frames cannot be CAN-FD.
• Split out CAN/CAN-FD data frames into separate types? Or keep as a flag? CAN-FD has special flags (e.g. BRS).
• Split out Error frame as separate type?
• Investigate use of [u8; 64] instead of Vec for CAN data storage