Simplify time form analysis

Simplify time form analysis

This does not change behavior; it merely refactors the code
for simplicity.
* src/timefns.c (enum timeform, struct form_time):
Remove.  All uses removed.
(decode_time_components): Accept HZ instead of FORM.
This saves a switch.  All uses changed.
(decode_lisp_time): Return union c_time instead of struct form_time.
All uses changed.
(lisp_time_cform): Remove.  All uses changed to just use
decode_lisp_time.
(time_arith, Ftime_convert): Check (TICKS . HZ) form directly
using CONSP, instead of using the old struct form_time.
That's fast enough here.

(cherry picked from commit e1089cc9b6d7abfe468175eb92b146c884e00374)