function stdev( a, b ) {
	return addon( a, b );
}
Returns the standard deviation 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} standard deviation

@example
var y = stdev( 0.0, 1.0 );
// returns ~0.354

@example
var y = stdev( 4.0, 12.0 );
// returns ~2.828

@example
var y = stdev( -4.0, 4.0 );
// returns ~2.828

@example
var y = stdev( +Infinity, 4.0 );
// returns NaN

@example
var y = stdev( -Infinity, 4.0 );
// returns Infinity

@example
var y = stdev( 1.0, NaN );
// returns NaN

@example
var y = stdev( NaN, 2.0 );
// returns NaN