Add support for UART to the NET chip

[bifurcation]

Fixes #140

To validate performance, I added a Ping and Pong packet types to the NET firmware, and logic to automatically respond to a Ping with a Pong. It works reliably with the modified NET firmware flashed, though it's a bit finicky as to when in the lifecycle of the NET run you send the Ping. I haven't gotten deep enough in the NET firmware to understand what's going on, but it's reliable enough that it seems like the basic UART logic works.

There is a non-trivial conceptual shift here, in that App::handle is now async. This works fine in the ui-stm32 crate, but is causing some trouble in the ui-tauri crate (problems with holding locks on global state across await boundaries). As a result, the UART functionality is not currently implemented in ui-tauri.

[bifurcation]

It turns out we don't really need the async in the transmit direction. If the blocking transmit turns out to be an issue, we can investigate async later.