Polarization of Λ and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"><mml:mover accent="true"><mml:mi>Λ</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:math> in 920 GeV fixed-target proton–nucleus collisions