Calix[4]arene-Based 1,3-Diconjugate of Salicylyl Imine Having Dibenzyl Amine Moiety (L): Synthesis, Characterization, Receptor Properties toward Fe2+, Cu2+, and Zn2+, Crystal Structures of Its Zn2+and Cu2+Complexes, and Selective Phosphate Sensing by the [ZnL]