[Cr(phen)(ox)~2]^-: a versatile bis-oxalato building block for the design of heteropolymetallic systems. Crystal structures and magnetic properties of AsPh~4[Cr(phen)(ox)~2]·H~2O, [NaCr(phen)(ox)~2(H~2O)]·2H~2O and {[Cr(phen)(ox)~2]~2[Mn~2(bpy)~2(H~2O)~2(ox)]}·6H~2O