Function approximate_multifactorial_digits 

pub fn approximate_multifactorial_digits(
    n: Integer,
    k: u32,
    prec: u32,
) -> Integer

Calculates the approximate digits of a multifactorial. This is based on the base 10 logarithm of Sterling’s Approximation.

Panic

Will panic if either n is non-positive, or if n is inf as a Float.

Algorithm adapted from Wikipedia as cc-by-sa-4.0