function skewness( a, b ) {
	return addon( a, b );
}
Returns the skewness of an arcsine distribution with minimum support `a` and maximum support `b`.

@private
@param {number} a - minimum support
@param {number} b - maximum support
@returns {number} skewness

@example
var v = skewness( 0.0, 1.0 );
// returns 0.0

@example
var v = skewness( 4.0, 12.0 );
// returns 0.0

@example
var v = skewness( -4.0, 4.0 );
// returns 0.0

@example
var v = skewness( 1.0, -0.1 );
// returns NaN

@example
var v = skewness( 2.0, NaN );
// returns NaN

@example
var v = skewness( NaN, 2.0 );
// returns NaN