- Can't use Invert action for tap key, use On/Off insted.

- current_layer indicates active layer at the time
- default_layer indicates base layer
- default_layer is used when current_layer is 0
- with this LAYER_BIT action works as overlay even if
  default_layer varies other than layer 0.

- stuck tap key after tapping. seq[mods(d), key(d), mods(u), key(u)]
- tap key doesn't work when fast type. [key(d), mods(d), key(u),
  mods(u)]