implement signalling nan

[michaeljclark]

currently, the code canonicalizes non-zero fractions of the infinity/nan exponent by setting the most significant bit of the mantissa (bit 3) i.e. nan(0x8) which is recommended to be the quiet bit. so it could be defined that (bit 3) i.e. nan(0x8) is the quiet bit so that a non zero mantissa with the quiet bit clear, such as (bit 0). such as nan(0x1) - nan(0x7) represents signalling nans.

The IEEE 754 standard make formal requirements and recommendations for the encoding of the signaling/quiet state.

For binary interchange formats, the most significant bit of the significand field is exclusively used to distinguish between quiet and signaling NaNs. Moreover, it should be an is_quiet flag. That is, this bit is non-zero if the NaN is quiet, and zero if the NaN is signalling.