Refactoring: three implementations + unit tests

The test script will only ouput mismatching clocks unless called with
the "debug" argument.

To compare the running time of versions, use:

    for s in $(ls --color=none bashtime_*.sh) ; do echo -e "\n$s"; time ./test.sh $s debug > /dev/null; done

Right now, the bashtime_lists.sh script is the fastest on my system.