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When converting from float to (TICKS . HZ) form, do the
conversion exactly.  When converting from (TICKS . HZ) form to
float, round to even precisely.  This way, successfully
converting a float to (TICKS . HZ) and back yields a value
numerically equal to the original.
* src/timefns.c (flt_radix_power_size): New constant.
(flt_radix_power): New static var.
(decode_float_time): Convert the exact numeric value rather
than guessing TIMESPEC_HZ resolution.
(s_ns_to_double): Remove; no longer needed.
(frac_to_double): New function.
(decode_ticks_hz): It is now the caller’s responsibility to
pass a valid TICKS and HZ.  All callers changed.
Use frac_to_double to round (TICKS . HZ) precisely.
(decode_time_components): When decoding nil, use
decode_ticks_hz since it rounds precisely.
(syms_of_timefns): Initialize flt_radix_power.
* test/src/timefns-tests.el (float-time-precision): New test.