Make time arithmetic more like comparison

Make time arithmetic more like comparison

Since time comparison says X == X, have time arithmetic behave
similarly for X - X.  This should also be a bit faster due to not
having to test for floats and NaNs.
* src/timefns.c (time_arith, time_cmp):
Simplify by not worrying about NaNs, which are not time values.
(time_arith): Simplify by not worrying about subtracting nil from
nil; the caller now handles this.
(Ftime_subtract): Handle subtracting X from X specially.