Design, Synthesis, and Biological Activities of Classical N -{4-[2-(2-Amino-4-ethylpyrrolo[2,3- d ]pyrimidin-5-yl)ethyl]benzoyl}- l -glutamic Acid and Its 6-Methyl Derivative as Potential Dual Inhibitors of Thymidylate Synthase and Dihydrofolate Reductase and as Potential Antitumor Agents 1