- 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"
