Implementation of factorial function in Scheme language.

It is based on the formula:

`n! = n * (n - 1)!`

and

`0! = 1`

This first implementation in a recursive way:

(define (factorial x) (if (= x 0) 1 (* x (factorial (- x 1)))))

This second implementation in a iterative way:

(define (factorial x) (define (fact-iter a product) (if (= a 0) product (fact-iter (- a 1) (* product a)))) (fact-iter x 1))