taken from Hasu codebase (https://github.com/tmk/tmk_keyboard) usb.c/.h
and usb_extra.c/.h, though these files only have the PJRC
copyright/license in the header
(plus a few small aesthetic things in /src/keyboard. i changed some
function like macros to lower-case, because someday they might be
implemented as real functions... and there's no real reason to
distinguish between functions and function like macros in the main() and
other higher level code. at least that's what it seems like to me right
now.)
- fixed some includes (`uint8_t` comes from a header, not the language)
- put code from some of the .h files into .c files
- now using open drain logic (hi-Z or GND) on both chips instead of
driving the row pins high on the teensy, or using a pull-up resistor
on the row pins with the mcp23018
- put `return 0;` at the end of some functions that weren't void
- fixed/updated some documentation; esp. the row assignments for the
mcp23018
- generalized the unused/row/column pin assignment and init code using
macros, so they'll be much easier to move around if necessary
- fixed a redefinition error in "lib/usb/keyboard-usage-page.h"
