[APFloat] Properly implement next for DoubleAPFloat
Rather than converting to the legacy 106-bit format, perform next() on the
low APFloat. Of course, we need to renormalize the two APFloats if
either of the two constraints are violated:
1. abs(low) <= ulp(high)/2
2. high = rtne(high + low)
Should renormalization be needed, it will increment the high component
and set low to the smallest value which obeys these rules.
