Adding basic USB implementation

[ryan-summers]

Adding a PR for basic USB terminal support.

For some reason, this isn't working yet on Windows (descriptor rquests are failing and windows can't recognize the device).

I suspect something's going wrong in the configuration of the peripheral and am still investigating.

Current USB interface is an echo chamber:

PS C:\Users\rsummers> python -m serial COM22
--- Miniterm on COM22  9600,8,N,1 ---
--- Quit: Ctrl+] | Menu: Ctrl+T | Help: Ctrl+T followed by Ctrl+H ---
echo: a
echo: s
echo: d
echo: f
echo: s
echo: a
echo: s
echo: d
echo: f
echo: s
echo: a

[ryan-summers]

The USB enumeration issue appears to be a result of rust-embedded-community/usb-device#128 - I'll submit a patch to usb-device to get this fixed.

[ryan-summers]

The intent is to follow-up with an actual settings interface in the future now that we have a USB interface, and then use the sequential-storage crate to store values in the unused flash. This supports #351