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.
