We now use a 32-bit structure to describe each key, and an `uhk_key_t` type that
wraps all the supported things into a union. But this is not the only change: to
be able to move the Fn/Mod keys anywhere, instead of hardcoding their location,
they are now proper layer keys. To make it easier to handle turning a layer off,
key releases can be handled explicitly, too.
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
