Move command_type to byte 4

All the examples I'm seeing in other ev3 code have the command_type as byte 4:

0-1 Size
2-3 Sequence Number
4 Command Type
5-6 Local/Global
7-N Commands

The code had the command_type coming after Local/Global. I'm not sure why it works both ways, but it seems to be worth making the switch. This moves the default command_type method up into Base, so Base can add the command_type in to_bytes.