Get significand and exponent of a floating point number.
Breaks the floating point number X into its binary significand SGNFCAND
(a floating point value between 0.5 (included) and 1.0 (excluded))
and an integral exponent EXP for 2, such that:
X = SGNFCAND * 2^EXP
The function returns the cons cell (SGNFCAND . EXP).
If X is zero, both parts (SGNFCAND and EXP) are zero. (fn X)