Versatile synthesis of stereospecifically labelledD-amino acids via labelled aziridines—preparation of (2R,3S)-[3-2H1]- and (2R,3R)-[2,3-2H2]-serine; (2S,2′S,3S,3′S)-[3,3′-2H2]- and (2S,2′S,3R,3′R)-[2,2′,3,3′-2H4]-cystine; and (2S,3S)-[3-2H1-] and (2S,3R)-[2,3-2H2]-β-chloroalanine