Fix mods with tap key but some problems still exists.

- 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)]

common/keyboard.h

@@ -26,6 +26,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 extern "C" {
 #endif
 
+// TODO: union {raw = row:col}
 typedef struct {
     uint8_t row;
     uint8_t col;