Stirling's Approximation

Provide 2 functions (tof64 and tobigdecimal) that calculates the factorial of a number using the Stirling's Approximation formula.

This formula let you calculate the factorial with a very good precision for big numbers, and is not recursive, which saves a lot of computation time.

The formula is the following: n! = sqrt(2 * PI * n) * (n / E) ^ n

Example

```rust use stirling_approximation;

let factorial = stirlingapproximation::tof64(10); let highprecisionfactorial = stirlingapproximation::tobigdecimal(10);

println!("The factorial of 10 is: {}", factorial); println!("The high precision factorial of 10 is: {}", highprecisionfactorial); ```