Additions for the calendar module.

library/calendar.m:
    Add predicates and functions for converting between 1-based
    and 0-based month numbers and values of the month/0 type.
    The former are useful when dealing with human-provided dates
    whereas the latter are useful when interacting with dates
    provided from some foreign language (C and Java for example).

    Add a predicate for testing for equality of dates ignoring
    their time component.

    Replace the locally defined predicate char_to_digit/2 with
    calls to char.decimal_digit_to_int/2

    Fix some formatting issues.

tests/hard_coded/calendar_test.{m,exp}:
    Extend this test case to cover the functionality added
    above.

NEWS:
        Announce the additions.