usb host keyboard: add keymap support

[jepler]

I'll let you decide whether to pull this into your PR, or would prefer me to PR this direct to adafruit after your PR's merged.

This is confined to pure ASCII, but does support Alt Gr. This is enough to make a French keyboard layout work:

• home row is "qsdfghjklm"
• numerals are all shifted
• keys like {} require Alt Gr

While not tested, this should also support layouts like Colemak with the necessary keymap file.

Layouts can be produced from @Neradoc's keyboard layout modules on the device & loaded at runtime. Code to do so is at
https://gist.github.com/jepler/597c0a00e0eb014a433e03c0ae363a08
as well as a sketch of how an extended format could support dead keys & the full Unicode BMP instead of just ASCII.

import keyboard_layout_win_fr
import layout_to_settings
import usb
lay = layout_to_settings.layout_to_settings0(keyboard_layout_win_fr.KeyboardLayout)
usb.set_user_keymap(lay)