function deg2rad( x ) {
	return addon( x );
}
Converts an angle from degrees to radians.

@private
@param {number} x - angle in degrees
@returns {number} angle in radians

@example
var r = deg2rad( 90.0 );
// returns ~1.571

@example
var r = deg2rad( -45.0 );
// returns ~-0.785

@example
var r = deg2rad( NaN );
// returns NaN