also, mcp23018_init() needs fixing: `twi_stop()` needs to be at the end
of transmission blocks. i wouldn't think that would be necessary, but
it seems to be the only thing that'll make it work, and it also seems
consistent with the protocol diagram in the datasheet (lol, imagine
that), so i think that's what i'll have to do. not as though it matters
much i guess, since it's a single master / single slave system anyway, i
was just hoping not to release the bus till i was finished..
- 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"
Ben Blazak