<chrono> support

• benri now supports interaction with the <chrono> library.
• In order to be forward-compatible with the C++20 version of <chrono>, the month and
  year types had to be be modified:
  • month_t is now calendar_month_t with a length of 2.592×10⁶ s.
  • year_t is now calendar_year_t with a length of 3.155 760×10⁷ s.
  • month_t maps now to the new gregorian_month_t with a length of 2.629 746×10⁶ s.
  • year_t maps now to the new gregorian_year_t with a length of 3.155 695 2×10⁷ s.
  • Because lightyear_t is a function of year_t it slightly changes as well.