Log Message:
nickle: update to 2.81.

    Update to version 2.81

    test: Can't test for Y2038 fix because 32 bit machines

    glibc on all 32-bit machines fails the Y2038 test. Change that to use
    the maximum possible 32-bit value instead (sigh)

    Remove unused variable in builtin-date.c

    Add date conversion functions localtime, gmtime, timelocal, timegm

    These just wrap the C versions, except they report actual years,
    rather than years since 1900 and January is month 1, not 0.

    Add JSON tests

    Add floats to JSON module.

    Support floating point values in JSON input and output.

    Make nickle-tutorial.pdf build reproducibly

    Set TeX dates to RELEASE_DATE.
    Remove PDF /ID entry.