Neutron Diffraction Structure of (2R,3R)-l-(−)-[2-D]Carnitine Tetrachloroaurate, [(CH3)3N-CH2-CHOH-CHD-COOH]+[AuCl4]-: Determination of the Absolute Stereochemistry of the Crotonobetaine-to-Carnitine Transformation Catalyzed by l-Carnitine Dehydratase from Escherichia coli