C-Stannylated Carbohydrate Derivatives. Part 5. 1,2:5,6-Di-O- isopropylidene-3-C-(organostannyl)- and -3-C-(phenylstannyl)methyl-. alpha.-D-allofuranose Compounds. X-Ray Crystal and Molecular Structure of 1,2:5,6-Di-O-isopropylidene-3