Returns the factorial of `n,' which should be a non-negative integer. The result will returned as a floating-point number, single-float if possible, otherwise double-float. If it is returned as a double-float, it won't necessarily be integral, since the actual computation is (exp (gamma-ln (1+ n))) Implementation is loosely based on Numerical Recipes in C, section 6.1. On the TI Explorer, the largest argument that won't cause a floating overflow is 170.