issnan function needs work.

[WarrenWeckesser]

From https://en.cppreference.com/w/cpp/types/numeric_limits/signaling_NaN,

In IEEE 754, the most common binary representation of floating-point numbers, any value with all bits of the exponent set and at least one bit of the fraction set represents a NaN. It is implementation-defined which values of the fraction represent quiet or signaling NaNs, and whether the sign bit is meaningful.

So the bit-twiddling done in https://github.com/WarrenWeckesser/ufunclab/blob/main/src/issnan/issnan_ufunc.c.src is not gauranteed to work.

When the code is updated, it should make use of the appropriate features of the standard library and any relevant macros provided by the compiler:

• std::numeric_limits<T>::has_signaling_NaN
• issignaling macro: GNU docs, Open-std docs