Move timestamp-related stuff to timefns.c

Move timestamp-related stuff to timefns.c

This does not change behavior; it’s just long-overdue
refactoring (Bug#32902).
* src/emacs.c (main): Call init_timefns, syms_of_timefns.
* src/timefns.c: New file, containing timestamp-related stuff
from editfns.c and sysdep.c.
* src/Makefile.in (base_obj): Add timefns.o.
* src/editfns.c: Simplify by moving a big chunk to timefns.c.
Do not include systime.h, sys/resource.h, sys/param.h,
strftime.h, coding.h.
(HAVE_TZALLOC_BUG, TM_YEAR_BASE, HAVE_TM_GMTOFF, tzeqlen)
(local_tz, utc_tz, emacs_localtime_rz, emacs_mktime_z)
(invalid_time_zone_specification, xtzfree, tzlookup)
(TIME_T_MIN, TIME_T_MAX, time_overflow, invalid_time)
(check_time_validity, hi_time, lo_time, Fcurrent_time)
(time_add, time_subtract, time_arith, Ftime_add)
(Ftime_subtract, Ftime_less_p, Fget_internal_run_time)
(make_lisp_time, disassemble_lisp_time, decode_float_time)
(lisp_to_timespec, lisp_time_struct, lisp_time_argument)
(lisp_seconds_argument, Ffloat_time, emacs_nmemftime)
(Fformat_time_string, format_time_string, Fdecode_time)
(check_tm_member, Fencode_time, Fcurrent_time_string)
(tm_gmtoff, Fcurrent_time_zone, Fset_time_zone_rule)
(emacs_getenv_TZ, emacs_setenv_TZ): Move to timefns.c.
* src/emacs.c (main): Adjust to initialization changes.
* src/sysdep.c: Include <sys/resource.h> if it's present.
Regularize includes a bit.
(Fget_internal_run_time): Move here from editfns.c.
(init_timefns, syms_of_timefns): New functions.
* src/w32.h (w32_get_internal_run_time): Move decl here
so that it need not be cloned.
* test/src/editfns-tests.el:
* test/src/editfns-tests.el (format-time-string-with-zone)
(format-time-string-with-outlandish-zone)
(editfns-tests--have-leap-seconds)
(format-time-string-with-bignum-on-32-bit):
Move to ...
* test/src/timefns-tests.el: ... this new file.