changed 'KEY__' to 'KEYBOARD__' for keycode macros
working on the layer-stack; finished! (for now)

- did not move any of the layer-stack code into code for a  'flex-array'
  type, or something similar.  i'm thinking that most of the things the
  layer-stack does aren't sufficiently generalizable. for instance, one
  of the biggest general functions in the layer-stack implementation is
  `_shift_elements()`; but shifing elements is only something you want
  to do when you're messing with elements not on the the top of the
  stack (which breaks the definition of a general stack).  so i think
  i'll leave things as they are for now.  the functionality can always
  be split out later if it turns out to be needed elsewhere.
(messing with makefiles; not working yet)
makefile working; doesn't compile yet