Rework example code, fix bugs and get to compiling state. Also introduce missing logic for the Matrix code to keep track of what column is active.

example/obsolete/multiplexer/Makefile

@@ -0,0 +1,8 @@
+TARGET_BOARD=arduino:avr:uno
+COM_PORT=/dev/ttyACM0
+
+build:
+	arduino-cli compile -b ${TARGET_BOARD}
+
+upload:
+	arduino-cli upload -b ${TARGET_BOARD} -p ${COM_PORT}