function factorial( x ) {
	return addon( x );
}
Evaluates the factorial of `x`.

@private
@param {number} x - input value
@returns {number} factorial

@example
var v = factorial( 3.0 );
// returns 6.0

@example
var v = factorial( -1.5 );
// returns ~-3.545

@example
var v = factorial( -0.5 );
// returns ~1.772

@example
var v = factorial( 0.5 );
// returns ~0.886

@example
var v = factorial( -10.0 );
// returns NaN

@example
var v = factorial( 171.0 );
// returns Infinity

@example
var v = factorial( NaN );
// returns NaN