Toward Rational Construction of Gold, Gold-Silver, and Gold-Mercury String Complexes: Syntheses, Structures, and Properties of [Au(Tab)2]2L2 (L = I and PF6), {[(Tab)2M][Au(CN)2]}2 (M = Au and Ag), and {[Hg(Tab)2][Au(CN)2]2} [Tab = 4-(Trimethylammonio)benzenethiolate]