Support larger TIMEs in (time-convert TIME t)

Support larger TIMEs in (time-convert TIME t)

Also, improve the doc to match current behavior.
* doc/lispref/os.texi (Time Conversion): Document that
time-convert signals an error for infinite or NaN args,
and that (time-convert TIME t) is exact otherwise.
Mention float-time as an alternative to time-convert.
(Time Calculations): Document that time-add and time-subtract
are exact and do not decrease HZ below the minimum of their args.
* src/timefns.c (decode_float_time): Don’t signal an error for
floating-point arguments whose base-FLT_RADIX exponent is not less
than DBL_MANT_DIG.  Instead, convert them to (TICKS . 1) values.
Use two (instead of three) integer exponent comparisons in the
typical case.
* test/src/timefns-tests.el (time-arith-tests):
Add more floating-point tests, including some tests
that the old code fails.