platform(d1): USB host mode driver

[hawkw]

this is probably a huge pain in the ass to get working, but imagine being able to use a Normal Keyboard with mnemOS. it would be especially nice for folks without a beepy board.

if we make a USB keyboard our MVP for a USB host driver, the Right Way to do this might look like:

• design a cross platform usb host Service abstraction
• implement that for the D1
• write a cross platform USB keyboard driver using the USB host Service and existing keymux stuff

Note: this issue is specifically about implementing USB host mode, for connecting other USB devices to a mnemOS system. support for mnemOS systems operating as a USB device is tracked by issue #88.