Implement Clulow's serial command protocol

VM commands are currently pretty sketch, given that we don't get basic nice things like return codes by virtue of executing commands through a serial interface. In a conversation I had a while back with @jclulow, he had a great idea for this.

Build a small rust binary, let's call it commander. When a falcon serial console session starts, the commander binary is sent to the guest file system through the serial console and started. The commander program then takes control of the TTY for the serial console. From that point forward, falcon and the commander communicate over a binary format such as postcard. When falcon sends a command, instead of it being executed in a shell session over the serial console, the binary representation is sent over the serial console, the commander program decodes it, executes the command inside by shelling out (which gives us a return code), and sends a binary response back over the console which includes the return code.

The issue names no files or tests; start by tracing Falcon's serial console session and VM command path, then examine the proposed commander binary and its TTY handoff. Done means commands use a binary serial protocol, execute in the guest with return codes, and receive binary responses.