[WIP] Add stand-in CommandProcessor with I2C...

Add manually-coded CommandProcessor with I2C forwarding capabilities.

I2C request forwarding
======================

A request with the type `ForwardI2cRequest` has two fields:

 - `address`: I2C address of device to forward request to.
 - `request`: Encoded protocol buffer request bytes.

The forwarding node _(i.e., the one who received the
`ForwardI2cRequest`)_ forwards the encoded request over the I2C bus to
the node with the specified address.  After receiving the response over
the I2C bus, the forwarding node returns the response over the serial
interface of the forwarding node.

Notes:

 - The forwarding node must be configured as master of the I2C bus.
 - Any node that will receive forwarded messages must be configured as
   slave devices on the I2C bus.
 - There are several possible mechanisms that we could use to allow any
   node on the I2C bus to forward requests to any other node on the bus:
  - Use a multi-master configuration with some mechanism for bus
    arbitration to allow any node to become the master while processing
    a request.
  - Use broadcast messaging on the I2C bus to allow any node to send a
    message that is received by all other nodes on the bus.  This would
    require custom parsing and addressing, since broadcasting
    circumvents I2C addressing entirely.
  - Still have only a single master of the I2C bus, but have the master
    periodically poll the slave nodes for any outstanding requests that
    they might want forwarded.