Add Arduino library and demo: turn an 8-bit or 16-bit timer/counter into a software callback timer library

[ElectricRCAircraftGuy]

Could be useful for filtering out 8 kHz ~ 16 kHz motor PWM throttle pulses, for instance, when trying to read the commutation frequency to measure RPMs in a sensorless brushless DC motor.

Obviously it would have many more uses than this too. For RAM allocation, consider mallocing during initialization only, then never again. That's a safe way to go about doing it. The other option would be via static global variables to pre-allocate a memory chunk for the underlying sorted list to store the timeout times.