Getting a feel for how the development experience is with this setup. With the previous idea of abstracting the request into a getSystemInfo() function on the Unit itself, the documentation for what to expect from GetSystemInfo either had to live in two places or be presented as a link to the canonical location. Neither felt great, so I think the caller can just call send(GetRequest()) themselves.
Also added the first "set" message which is capable of toggling a body or circuit.
