I2C TOML: Hubris/Humility flag day

[bcantrill]

My recent push (#238) constitutes a Humility/Hubris flag day: if/when you update to latest Hubris, you will also need latest Humility (v0.5.0) in order to be able to use i2c-related Humility commands. The failure modes of not doing this will be relatively clean (i.e., things will fail but shouldn't crash), but the details depend on the mismatch.

If running an old Humility with a new Hubris, i2c-related HIF operations will fail. Some failures will be type errors:

$ humility i2c -s -c 4 -p f -m 1:3
humility: attached via ST-Link
humility: i2c failed: expected port to be an enum

Caused by:
    expected enum GOFF 0x00007770 (object 13) not found

But if a port isn't specified, it might appear a little weirder:

$ humility i2c -s -c 1
humility: attached via ST-Link
humility: i2c failed: request failed: Some(Fault(EmptyParameter(0x1)))

This will be true for other i2c-based commands as well (e.g., pmbus, spd, rencm), which will have similar failure modes.

If running a new Humility with an old Hubris, the failure mode will be much more explicit:

$ humility i2c -c 4 -p f -m 1:3 -d 0x60 -s
humility: attached via ST-Link
humility: i2c failed: no I2C buses found; is this an old Hubris image?

Of note, running humility manifest will show no controllers or devices on an old Hubris.

Please let me know if you hit any unexpected issues!