Add utility functions to CAN library

[Levi-Lesches]

There's some helpful code in arm-firmware that could be used in the other firmware repositories:

• split a number < 65535 into two bytes that can be sent separately

  
  struct split_uint16 {
  char byte1;
  char byte2;
  }

split_uint16 split_int(unsigned short num);

- Recombine two bytes into a `short` (two bytes)
```cpp
unsigned short recombine(char a, char b);

• Construct a CAN ID from a Node ID and a Command ID

  short getCanID(char nodeID, char commandID);