renamed, to be a little more consistent (i hope)
(...)
added todo
rewrote the layer-stack implementation

i was going to pull the functionality out into a "flex-array" type, and
use that, but it turned out not to generalize very well.  in the
process, i think i found a cleaner (and smaller) way to do what i was
already doing, so i rewrote the implementation.  i'm planning to rewrite
everything currently using linked-lists to use this type of dynamically
sized array, eventually, in order to save SRAM, but we'll see how that
goes.
(minor aesthetic)
added error checking and stopped implicitly converting -1
changed resize_stack() slightly
(minor: formatting)
(messing with error checking in resize_stack() again)
switched lib/eeprom from linked lists to dynamic arrays