Minor fixes/simplifications to time functions

Minor fixes/simplifications to time functions

* doc/lispintro/emacs-lisp-intro.texi (Files List): Simplify.
* doc/lispref/os.texi (Time of Day): Mention format-time-string
as an alternative to current-time-string.
* lisp/arc-mode.el (archive-unixdate, archive-unixtime):
Port better to future versions of Emacs where (COUNT . HZ)
will take precedence to (HI . LO).
* lisp/arc-mode.el (archive-unixtime):
* lisp/calendar/todo-mode.el (todo-insert-item--basic)
(todo-item-done, todo-read-time):
Prefer format-time-string to substringing current-time-string.
* lisp/calc/calc-forms.el (calc-time, calcFunc-now):
Prefer decode-time to parsing the output of current-time-string.
* lisp/emacs-lisp/cl-extra.el (cl--random-time):
Prefer encode-time to hashing the output of current-time-string.
* lisp/gnus/gnus-score.el (gnus-score-headers)
(gnus-score-adaptive):
Avoid stringifying and then reparsing timestamp.
* src/timefns.c (Fencode_time): Omit redundant assignment.