This was incorrectly translated as a u64. binary_exponent is an
unadjusted value so can be negative. In becomes unconditionally positive
when adding the bias.
This is not intended to be the long-term implementation as it doesn't
provide various properties that we eventually will want (e.g.
round-tripping, denormal support). It also uses f64 internally so the
wider f128 will be inaccurate.
