Fixed IOBuffer constructor; removed compiler warning.
Refactoring Pins; External/InterruptPin moved to separate files.

1. Ripple effect on drivers that use these classes.
2. Fixed counter in OutputPin::pulse(value, us).
Adding high/low to univ16_t/univ32_t for access of MSB/LSB.
Improving RTC and Watchdog. New test levels in RTC benchmark.

1. Removing RTC millis counter. Reusing ticks instead with adjustment.
2. Cleaning up benchmark.
3. Improving Watchdog await tick count calculation.
4. New Watchdog function; get_millis() as a complement to get_ticks().
Testing additional power down of ATtiny internal modules.