To make their purpose clearer, rearrange our state: we now have the row-based
array on the top level, instead of every member being an array on its own. The
name of the state variable was changed to `matrix_state_`, to reflect its
purpose. This also allowed us to have its members be named `current`,
`previous`, `debouncer` and `masks`.
All devices using these APIs, and the documentation were updated accordingly.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>