Add step sequencer feature (#9703)

* sequencer: create togglable feature

* sequencer: add support for steps

* sequencer: add support for tempo and resolutions

* sequencer: schedule a message print at the right frequency

* sequencer: send a hardcoded note

* sequencer: add support for 8 tracks

* sequencer: play several notes simultaneously

* sequencer: only play the active tracks for a given step

* sequencer: change the default behavior of the track toggler

* sequencer: make number of tracks and track notes customizable

* sequencer: move the keycodes down

Not adding them at the end of the list apparently risks breaking
compatibility with VIA.

Source: https://github.com/qmk/qmk_firmware/pull/9703#discussion_r459202733

* sequencer: add unit tests

* sequencer: add unit test for matrix_scan_sequencer

* sequencer: expose internal state for better unit-testability

* sequencer: add unit tests for matrix_scan_sequencer

quantum/process_keycode/process_sequencer.h

@@ -0,0 +1,21 @@
+/* Copyright 2020 Rodolphe Belouin
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "quantum.h"
+
+bool process_sequencer(uint16_t keycode, keyrecord_t *record);