* 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
Resolve the issue with the UnixHostDuino not really being compatible
with the esp8266 Core String (...and the rest of the Core, as well)
Port the CMakeLists.txt from the rpnlib and update it use FetchContent
instead of either manually fetching dependencies or using PIO artifacts
Caching is *expected* to work, but might need slight adjustments
