Binding and Removal of Sulfate, Phosphate, Arsenate, Tetrachloromercurate, and Chromate in Aqueous Solution by Means of an Activated Carbon Functionalized with a Pyrimidine-Based Anion Receptor (HL). Crystal Structures of [H 3 L(HgCl 4 )]·H 2 O and [H 3 L(HgBr 4 )]·H 2 O Showing Anion−π Interactions