function stdev( p ) {
	if (
		isnan( p ) ||
		p < 0.0 ||
		p > 1.0
	) {
		return NaN;
	}
	return sqrt( ( 1.0-p ) * p );
}
Returns the standard deviation of a Bernoulli distribution.

@param {Probability} p - success probability
@returns {NonNegativeNumber} standard deviation

@example
var v = stdev( 0.1 );
// returns ~0.3

@example
var v = stdev( 0.5 );
// returns 0.5

@example
var v = stdev( 1.1 );
// returns NaN

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