Thermodynamic Model for BiPO4(cr) and Bi(OH)3(am) Solubility in the Aqueous Na+–H+–$\mathrm{H}_{2}\mathrm{PO}_{4}^{-}$–$\mathrm{HPO}_{4}^{2-}$–$\mathrm{PO}_{4}^{3-}$–OH−–Cl−–H2O System