Avoid some excess precision in time arithmetic

* doc/misc/emacs-mime.texi (time-date):
Adjust example to match new behavior.
* etc/NEWS: Mention this.
* lisp/calendar/time-date.el (decoded-time-add)
(decoded-time--alter-second):
Don’t lose underestimate precision of seconds component.
* src/bignum.c (mpz): Grow by 1.
* src/timefns.c (trillion_factor): New function.
(timeform_sub_ps_p): Remove.
(time_arith): Avoid unnecessarily-large hz, by reducing the hz
to a value no worse than the worse hz of the two arguments.
The result is always exact unless an error is signaled.
* test/src/timefns-tests.el (timefns-tests--decode-time):
New function.
(format-time-string-with-zone): Test (decode-time LOOK ZONE t)
resolution as well as its numeric value.
1 job for master in 62 minutes and 4 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
failed #3050
test-all

01:02:04

 
Name Stage Failure
failed
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1