function cbrt( x ) {
	return addon( x );
}
Computes the cube root of a double-precision floating-point number.

@private
@param {number} x - input value
@returns {number} cube root

@example
var v = cbrt( 64.0 );
// returns 4.0

@example
var v = cbrt( 27.0 );
// returns 3.0

@example
var v = cbrt( 0.0 );
// returns 0.0

@example
var v = cbrt( -9.0 );
// returns ~-2.08

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