Stabilization of Noncondensed (As III S 3 ) 3– Anions by Coordinating to [Mn II (phen)] 2+ Complex Cations: a Mixed-Valent Thioarsenate (III, V) {[Mn(phen)] 3 (As V S 4 )(As III S 3 )} n · n H 2 O Showing the Coexistence of Antiferromagnetic Order, Photoluminescence, and Nonlinear Optical Properties