Asymmetric Synthesis of [2,3-13C2,15N]-4-Benzyloxy-5,6-diphenyl-2,3,5,6-tetrahydro-4H-oxazine-2-one via Lipase TL-Mediated Kinetic Resolution of Benzoin: General Procedure for the Synthesis of [2,3-13C2,15N]-l-Alanine