create spec for can bus communication

[pfrommerd]

We need to figure out how ids will be divvied up on the can bus. Ideally, this would allow for:

1. Resistance to variable id conflicts
2. Equivalent functionality to uart protocol/interface
3. Subscriptions, but not duplication of data (so that if two people on a bus subscribe to a variable, it gets sent out once, at the higher frequency)
4. Gracefully handling devices going offline in case of watchdog/fault happening.