BigFraction(double) constructor does not treat subnormal numbers correctly

The constructor BigFraction(double) does not take into account the fact that, when the biased exponent of a double value is 0 and the mantissa is not 0 (i.e. when the value represents a subnormal number), the actual exponent in effect is not -1023 but -1022 (or, in other words, the effective exponent bias is not 1023 but 1022).

The value of the created BigFraction is therefore not equal to the value of the passed double argument.

Also, since the constructor does not handle the case of zero separately, it creates a fraction with a numerator of 0 and a denominator of 2¹⁰⁷⁵, which is not very memory-efficient.