This continues the work of#1055.

• Working RTC peripheral on all STM32 except STM32F1, which has a completely different RTC
• Efficient and cached conversion algorithm to milliseconds since 1970 epoch.
• C++std::chrono interface for clocks.
• Add interface code for DateTime objects
  • There should be amodm::DateTime class that deals with conversions compatible withRtc::DateTime.
  • Construct from__TIMESTAMP__ instead of adding an extra class.
  • Unit tests for time conversion
• Better Board support for SystemClock. Lots of boards have an RTC crystal.
• Generic example showing off millisecond time_points and DateTime object.
• Tested on ~40 nucleo and discovery boards I had lying around with the generic example, covering almost all families.
• Ran unit tests on 5 STM32 boards.