Branches: main

Add predicates to the standard library's calendar module for folding over a
range of dates.

library/calendar.m:
   Add the new predicates: foldl_days/5, foldl2_days/7 and
   foldl3_days/9.

NEWS:
    Announce the addition.

tests/hard_coded/Mmakefile:
tests/hard_coded/fold_days.{m,exp}:
    Test the new predicates.