* globally accessible system timer class; help out with our internal
scheduling by always using strongly typed duration and dynamic
time adjustment for durations longer than system limits
(see `os_timer_t` documentation)
* our own class for scheduled callbacks which are either choosing a
simple function pointer or std function wrapper, depending on the type
passed into the constructor. specifically for scheduled functions,
this allows us to filter globally scheduled functions and push them to
the front of the queue when necessary to ensure certain order of calls
* replace Ticker instances with SystemTimer
* allow types {h, cpp} in unit tests