Add comment on time value formats. Don't require parse-time.

Add comment on time value formats.  Don't require parse-time.
(with-decoded-time-value): New macro.
(encode-time-value): New function.
(time-to-seconds, time-less-p, time-subtract, time-add): Use them.
(days-to-time): Return a valid time value when arg is huge.
(time-since): Use time-subtract.
(time-to-number-of-days): Use time-to-seconds.