function poisson( rand, lambda ) {
	if ( lambda < 30.0 ) {
		return knuth( rand, lambda );
	}
	return rejection( rand, lambda );
}
Returns a pseudorandom number drawn from a Poisson distribution with parameter `lambda`.

@private
@param {PRNG} rand - PRNG for generating uniformly distributed numbers
@param {PositiveNumber} lambda - mean
@returns {NonNegativeInteger} pseudorandom number