function signum( x ) {
	return addon( x );
}
Evaluates the signum function for a double-precision floating-point number.

@private
@param {number} x - input value
@returns {number} function value

@example
var sign = signum( -5.0 );
// returns -1.0

@example
var sign = signum( 5.0 );
// returns 1.0

@example
var sign = signum( -0.0 );
// returns -0.0

@example
var sign = signum( 0.0 );
// returns 0.0

@example
var sign = signum( NaN );
// returns NaN