Allows other plugins to request a slice of EEPROM, and returns the starting
location of their area. Makes a CRC out of the slice sizes, so that it can
detect when the EEPROM and the Sketch become out of sync. Handling that case is
left up to the user.
As a consequence, we no longer reserve a big chunk of EEPROM for the keymap,
that just becomes another slice of it, which can be anywhere. This makes it a
bit harder to adjust the size of it, but as far as this plugin goes, playing
with the EEPROM layout will usually mean having to update its contents from
scratch, anyway.
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>