This fixes a bug with overlapping sticky keys when the same key
positions on multiple layers contain different sticky keys.
This also fixes the case when a MT has a sticky key for hold and a
different sticky key for the tap behavior.
Fixes#508
this makes LS(LEFT_CONTROL) work as if shift and control were both
pressed explicitly. Previously, the left shift would have been released
as soon as another key was pressed. The implicit behavior is useful in
case of LS(NUMBER_1) when rolling over to other keys.
Also see #361.