Inhibitors of Glycinamide Ribonucleotide Formyltransferase as Potential Cytotoxic Agents. Synthesis of 5-Deaza-5,6,7,8-tetrahydrohomofolic Acid, 5-Deaza-5,6,7,8-tetrahydroisohomofolic Acid, and 10-Formyl-5-deaza-5,6,7,8-tetrahydroisohomofolic Acid