Interactions of Tubulin with Guanine Nucleotides That Have Paclitaxel-like Effects on Tubulin Assembly: 2′,3′-Dideoxyguanosine 5′-[α,β-Methylene] Triphosphate Guanoine 5′-[α,β-Methylene] Triphosphate, and 2′,3′-Dideoxyguanosine 5′-Triphosphate