Function estimate_scaling_factor

pub fn estimate_scaling_factor(mant: u64, exp: i16) -> i16

Finds k_0 such that 10^(k_0-1) < mant * 2^exp <= 10^(k_0+1).

This is used to approximate k = ceil(log_10 (mant * 2^exp)); the true k is either k_0 or k_0+1.