function arcsine( rand, a, b ) {
	return a + ( pow( sin( HALF_PI*rand() ), 2.0 ) * ( b-a ) );
}
Returns an arcsine distributed pseudorandom number with minimum support `a` and maximum support `b`.

@private
@param {PRNG} rand - pseudorandom number generator
@param {number} a - minimum support
@param {number} b - maximum support
@returns {number} pseudorandom number